The need to boost student learning in the wake of the pandemic has made school leaders’ jobs more challenging and crucial than ever. Two companion studies synthesizing 20 years of research on principals and assistant principals reveal just how much the most effective principals add to student achievement, how they do it, and the special role that assistant principals can play in increasing equity in schools.

Principals can have an even greater impact on student achievement than previously realized, according to a 2021 Wallace Foundation report, “How Principals Affect Students and Schools: A Systematic Synthesis of Two Decades of Research.” Analyzing research published since 2000 that tracked the progress of the same schools under different leaders, researchers found that principals in the 75th percentile of effectiveness added almost three months of learning per year in math and reading on standardized tests compared with principals in the 25th percentile.1 That’s nearly as much as the four additional months’ worth of learning each year that previous research attributed to teachers in the 75th percentile versus those in the 25th percentile.

But while teachers affect achievement only for their own students, principals impact learning schoolwide, says Jason Grissom, the Wallace study’s lead researcher. “Where you have a strong principal, you have the opportunity for big effects.”

Here’s what Grissom’s synthesis finds are the most effective strategies principals use:

Engage with teachers about instruction. Principals who have positive effects on student achievement make time to observe classroom instruction and provide teachers with specific, useful, actionable feedback. In middle and high schools, that could mean leveraging the expertise of a leadership team that includes assistant principals, lead teachers or department chairs, and instructional coaches with a variety of expertise.

The most effective principals also link professional development to needs identified during classroom observations and school goals. This could include arranging for individual coaching for teachers on instruction or behavior management.

Finally, the best principals create systems for the strategic use of data. Such systems can include “data chats” between administrators and teachers that allow them to reflect together on student data, strengthen instruction, monitor student progress, and target interventions for students.

One surprising finding is that “walkthroughs,” or a series of brief visits to classrooms to gauge practice, are probably not worth your time, Grissom says. While some principals do use the data gathered during walkthroughs, on average, they do not. Grissom’s own study from 2013 even found that the least effective principals spend the most time on walkthroughs, possibly because they spend less time on more worthwhile instructional leadership tasks.

Establish a productive school climate. The bedrock of a school climate that accelerates student learning is caring, respectful relationships—and forging those relationships begins at the top. When teachers feel emotionally safe and supported, they are more willing to work together on improving instruction.

One initial step to building relationships supported by research is conducting a needs assessment that identifies a school community’s existing values, priorities, needs, and norms. Principals also need to get to know teachers individually to understand their expertise, personality, and needs.

Empowering teachers is another way to build trust. Research-supported strategies include promoting teacher leadership, offering mentoring and professional development, facilitating collaboration, and providing timely data on students that teachers can use to plan instruction.

Facilitate collaboration. Many schools schedule time for teachers at the same grade level or within the same department to work in professional learning communities (PLCs). Ideally, teachers use the time to share successful instructional strategies, analyze student work or test results and discuss how teaching can improve, and perhaps arrange to informally observe each other and provide feedback. But many PLCs fall short of this idea, Grissom says. “Teachers often do not like them or find them productive.”

PLCs are more likely to boost student achievement when they are formally structured and school leaders set clear expectations for what they should accomplish, he explains. An assistant principal or department chair may need to facilitate the meeting or at least model facilitation and check in periodically to ensure that it’s productive, Grissom says. “You have to implement it like any other program in your school.”

Manage resources and personnel strategically. Effective management—of facilities, the school budget, school safety, etc.—is associated with student achievement growth. But research finds that one of the most valuable resources for principals to manage is their own time. Principals know they should spend more time on instruction but typically don’t follow through as more pressing issues arise, Grissom says. The most effective principals create systems to prevent intrusions on their time so they can focus on instruction.

Managing personnel effectively is also strongly correlated with student achievement gains. Here are a few key strategies:

  • Hire based on rich information about a teacher’s ability to raise achievement. This might include asking a candidate to teach a demonstration lesson, respond to questions based on case studies, or converse with multiple staff members, including experienced teachers able to gauge the candidate’s thinking about instruction. Traditional interviews, by contrast, can be weak predictors of teaching competence.
  • Place the strongest teachers with the lowest-achieving students.
  • Retain the highest-performing teachers by building trusting relationships and a productive school climate.

The Rise of the Assistant Principal

School leadership is too complex a job to go it alone. Given the mounting expectations for school leaders, it’s perhaps not surprising that the number of assistant principals has skyrocketed in recent decades, according to another 2021 Wallace Foundation Report, “The Role of the Assistant Principal: Evidence and Insights for Advancing School Leadership.” Between 1990 and 2015, the number of assistant principals grew from 44,000 to 81,000 nationally—a remarkable 83% increase.

“It’s a counter-narrative to the idea that if you want to improve schools, all the resources need to be in the classroom,” observes Ellen Goldring, one of the report’s lead authors.

Although she says research isn’t clear on the degree to which assistant principals are able to impact student achievement or school improvement, a handful of studies included in the report point to areas of potential influence such as training assistant principals to coach teachers to improve instruction or having them be visible in classrooms to reduce disciplinary incidents.

Unfortunately, assistant principals spend less than half their time on tasks related to instructional leadership, her research synthesis found. That has implications for student learning now and in the future. Most assistant principals go on to become principals, and those who do name their experience as an assistant principal as the most valuable part of their preparation and mentoring from their principal as key.

“The takeaway here,” says Goldring, “is that principals have a really important role in working with assistant principals to make them highly effective.”

The two Wallace studies also pointed to areas where assistant principals could play an important role in promoting equity.

Student discipline. Principals wield great influence over school suspension and expulsion rates, Grissom’s study found. And students of color are disproportionately subjected to discipline that excludes them from school, which has long-term consequences for achievement and employment. Since student discipline is a common responsibility for assistant principals, Goldring’s study noted, training them on equitable discipline and culturally responsive approaches could reduce the use of harsher measures.

Family engagement. Where parental involvement in schools is high, Grissom’s study documented, student achievement is higher and disciplinary infractions are lower. In a large school, building those relationships can’t be the principal’s responsibility alone, he says. Touchpoints for parent engagement can include open houses, arts, athletics, individual conferences with teachers or administrators, and international-themed events that draw on the cultural backgrounds of families. “What’s important is that there are leaders in the building that are engaged in outreach to bring a diverse range of parents into the school, not just the ones who always show up,” Grissom says.

Diversifying Leadership

The two Wallace studies found that administrators of color are underrepresented in the profession. Principals and assistant principals, however, can play a key role in increasing diversity. And research shows that having principals with diverse backgrounds can result in better outcomes for students of color.

While the percentage of principals of color has risen in recent decades, according to Grissom’s study, it has not kept pace with the increase in student diversity. By 2016, 8% of principals were Hispanic, 11% were Black, and 79% were white. Meanwhile, 23% of public school students were Hispanic, 15% were Black, and 53% were white.

Assistant principals of color were also found underrepresented compared to the student population. Goldring’s study reviewed research that suggests racial bias and inequities in access to mentoring likely played a role in slowing advancement to the principalship. Adding to the inequity in preparation, some research also found that assistant principals of color are spending more time on student discipline (suggesting they spend less time on instructional leadership than white principals).

The representation gap in school leadership hurts student achievement. According to Grissom’s study, principals of color are more likely to hire and retain teachers of color, and teachers of color are more likely to hold higher expectations for students of color. Black and Hispanic students are also more likely to be placed in advanced classes or gifted programs when the principal is Black or Hispanic. (The study found evidence that white principals were most likely to impact achievement for students of color when they made a concerted effort to do so, such as by learning about race, engaging staff on racial issues, and disaggregating student data by race to address disparities in outcomes.)

Assistant principals and principals can play a crucial role in closing the representation gap by encouraging teachers of color to become assistant principals and principals, Goldring says. And that encouragement needs to be proactive and planned as opposed to being given as casually offered advice. “The literature talks about the importance of ‘tapping’—the principal noticing a teacher’s leadership abilities and interest and helping them navigate the pathway to the principalship.”

That support, she says, can include an invitation to take on teacher leadership roles and being alerted to leadership development opportunities, being mentored by an assistant principal, and being coached on how to navigate licensure and preparation.

Grissom and Goldring predict that in the years ahead, principals and assistant principals will play increasingly important roles in promoting educational equity. “That’s the direction we’re collectively heading,” Grissom says.” We’re going to be looking to school leaders for solutions. We’re going to be looking to them to help lead that charge.”

Elizabeth Duffrin is a freelance writer and editor based in Chicago.


1 Grissom’s study builds on a similar 2004 analysis finding that principal leadership is second only to teaching in school-related factors impacting student achievement. A recent working paper posted on Brown University’s Annenberg Institute website challenges a statistical method used by Grissom and others to quantify the impact of an effective principal on student achievement, concluding the method may misattribute gains in student performance to principals. The paper does not challenge other research linking effective principals, and their improved preparation and support, to positive impacts on schools, staff, and students.


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