Are Good Schools Simply a Collection of Good Teachers?
This question is actually reframed from a line in Richard Allington's and Patricia Cunningham's resource Schools That Work: Where All Children Learn to Read and Write, 3rd Ed. (Pearson, 2007). I was looking through this text as I prepared a site license of my book for another school. Allington and Cunningham devote a couple of pages on types of portfolios that can be used in schools for more authentic assessment. Below is the larger quote in which this question is derived. It is one of five "truths" they have uncovered after a combined 50 years of experience in elementary schools:
Good schools are collections of good teachers, and creating schools where all children become readers and writers is simply a matter of figuring out how to support teachers in their efforts to develop the expertise needed to foster the reading and writing proficiencies of every student. (318)

The authors do address the role of the building administrator in their resource. For example, they advocate for principals to support Teachers as Readers book study groups, as well as creating their own literacy networks (Administrators as Readers). Also, the authors describe situations in which a school principal has gone above and beyond normal practice to advocate for students, such as supervising a morning study center and making visits to homes of students who have missed a lot of school. I've applied their suggestions to my own practice.
Here's the thing: I am not sure if these activities have made any kind of large impact on student learning, either in academic outcomes or in student dispositions toward learning. Furthermore, while I believe that modeling good instruction in literacy can have an influence on teacher practice, for those educators that may not employ best practice, they will likely not change their instruction in light of this type of leadership. Change in actions comes from a change in beliefs. And changing beliefs can only occur when the person that needs to improve is open-minded and has a desire to grow.
That is why I believe the principalship deserves a bit more focus when authors and experts consider whole school improvement initiatives. Recent resources have highlighted the importance of the principal. In Read, Write, Lead: Breakthrough Strategies for Schoolwide Literacy Success (ASCD, 2014), Regie Routman devotes a whole chapter of her text to "Leadership Priorities". She makes specific suggestions, such as leading like a coach (193) and making instructional walks a daily practice (197). Her own experiences and her reading of the research leads her to the conclusion that "school leadership matters as much as teacher quality" (181).
Baruti Kafele, a long time school principal, arrived at a similar conclusion. In his book The Principal 50: Critical Leadership Questions for Inspiring Schoolwide Excellence (ASCD, 2015), Kafele also promotes active leadership, encouraging school leaders to be present in classrooms daily and help create coherence in instructional practice throughout the school. He believes that the responsibility for the success of a school resides firmly on the shoulders of the principal.
When the principal can maintain the attitude that his or her overall leadership determines the success or failure of the school, students will benefit greatly. As I like to say, "Show me a school with extraordinary teachers in every classroom but an ineffective principal and I'll show you an underperforming school." (9)
What all I have shared here does not contradict what Allington and Cunningham have found in their own observations and studies. However, I do believe that up until recently, not enough attention was devoted to improving the capacities of school leaders. Even today, there is too much of a focus on trying to measure individual teachers' practices at the state level, and not enough attention on how to accurately and authentically assess the level of effectiveness a school might have on student learning. The outcomes of these results should largely be attributed to the leader of a school.
With that, I'd like to suggest a slight update to Allington's and Cunningham's position:
Good schools are collections of the right teachers.
What is bolded is what I changed, for a number of reasons.
There are a lot of good teachers out there, but not all of them may be a good fit for a school, team or department.
I'm thinking of a few superstars who by their own right (and probably their own admission) are fantastic at what they do, but are unable and/or unwilling to share their expertise with others for a variety of reasons. Maybe they are afraid that colleagues will take credit for something they developed. Maybe they take pride in having their students outperform other students, even within the same building. Whatever the reason, they can be toxic within a learning community and disrupt a previously positive environment. This can have a negative on a school's collective instructional impact, even if the kids in that teacher's classroom succeed. That's not okay.
Who a teacher is today is not necessarily who they might be tomorrow.
If they have a willingness to grow and better themselves as a professional, we as leaders have an obligation to support them in their self-improvement. Simply hiring our way toward an excellent school and/or getting rid of the less effective teachers isn't as easy nor as effective as others may lead you to believe. I'd much rather work with someone who is willing to better themselves than the alternative. In fact, I'd rather hire a professional with room to grow and a learner mindset than someone who is a superstar but is unwilling to work with colleagues for the betterment of the organization. Teaching is a team sport.
Sometimes a teacher is just in the wrong position.
Like a coach of a sports team who moves a third baseman to the outfield, a school leader can and should assign teachers to where their knowledge, skills and disposition are the best match. This can be assessed through frequent instructional walkthroughs, student assessment results, and observation of staff during professional collaboration activities. Principals who acquiesce their teacher assignment responsibilities give up one of their most powerful tools for creating grade level or department teams that have a high level of trust and effectiveness. When making these changes, I always try to highlight the positive attributes of the teacher as part of the rationale for this change.

I believe a good school is more than just a collection of good teachers. A good school is developed by bringing together a team of educators with diverse interests, skills, and backgrounds. Yes, they are good teachers, and can also direct their own professional learning, work collaboratively with colleagues, and engage families in school activities. This occurs through the thoughtful and intentional work of a principal. To be fair, I've searched for follow up work by Allington and Cunningham that addresses this essential element of schools that work for all students. So far, I haven't found anything. If they were to provide a 4th edition of their essential text, I hope they consider this area.