JEFFREY BROWN:And now to another in our series on the nation's high school dropout crisis, told this time through a different lens. Good teachers can help keep kids in school, but how can schools hold on to their top teachers?

Hari is back with a report for our American Graduate project.

HARI SREENIVASAN:It's a simple question at the center of almost any discussion on education reform. What makes a good teacher?

But the answers are many and often complex, and the question can lead to highly polarizing debates over exactly how and how often teachers should be evaluated on their job performance.

At the front lines of this debate are charter schools, the fastest-growing sector of American education, with two million students now enrolled in more than 5,000 such institutions across the U.S.

Bridgeport Academy Middle School in Bridgeport, Conn., is one of them. Like traditional public schools, it receives per-pupil funds from the state of Connecticut, but is allowed to operate independently from the local district and uses a blind lottery for enrollment. The school is part of the larger nonprofit Achievement First network of 22 charters along the East Coast serving mainly low-income minority students.

HARI SREENIVASAN:At Bridgeport Academy Middle School, the ultimate goal is to close the so-called achievement gap between rich and poor students. Its principal is Morgan Barth.

MORGAN BARTH, Bridgeport Academy Middle School: Bridgeport is on the bottom end of Connecticut, which has the biggest achievement gap in the country. And our kids are great, and they come to school with some really heartbreaking deficits in their academic skills. On average, a fifth grader comes to our school at least two or three grade levels behind.

HARI SREENIVASAN:These kinds of educational deficits have caused lingering problems for a city where one-third of all students fail to graduate on time. That has led to a concentrated effort by Bridgeport Academy Middle School, like all our Achievement First schools, to place and keep great teachers in the classroom.

In order to identify who those great teachers are, Achievement First CEO Dacia Toll says the organization has developed a comprehensive checklist to evaluate its teachers.

DACIA TOLL, Achievement First: In the past, teacher evaluation has focused on observations, which, at their worst, become staged dog and pony show experiences that don't actually tell you a lot about teachers' effectiveness or even more importantly how they need to improve.

WOMAN: I want us to now just walk quickly through the schoolwork for the whole observation.

HARI SREENIVASAN:At this, school teacher observations are detailed biweekly and discussed at length in regular coaching sessions.

WOMAN: You could have moved a little bit more efficiently through just the process itself.

HARI SREENIVASAN:And, perhaps surprisingly, they're also welcomed here.

JUDY ANDREWS, Teacher: I thought that my pacing was off a little bit. I thought that there were a couple times where I wasn't quite there as I normally am.

HARI SREENIVASAN:Judy Andrews is a fifth grade math teacher and was a veteran in the nearby New Haven Public School District until coming to Achievement First five years ago.

JUDY ANDREWS: The teaching model at Achievement First is different than the teaching model at most other schools in terms of the way the lessons are set up. So, I found it was extremely beneficial initially because I was getting immediate feedback.

HARI SREENIVASAN:Teachers here find out every day how effectively they have taught a lesson with short quizzes called exit tickets.

WOMAN:For your exit ticket, you have four problems.

HARI SREENIVASAN:Students are required to take one at the end of each class to measure their level of comprehension. But test scores and the results of classroom observations are not the only factors here that determine the quality of a teacher.

Again, Dacia Toll:

DACIA TOLL: We have parent, student, peer and principal surveys, so the teacher is really getting a whole 360 take on what they are doing well and what they need to improve.

HARI SREENIVASAN:A large part of the Achievement First plan to keep excellent teachers and those with potential to become great is the teacher career pathway.

It's a model designed to place classroom educators into five career stages depending on experience and classroom effectiveness, beginning at intern and concluding at master teacher. Reaching the fifth and final stage would bring a significant pay increase, although exact compensation figures are still being worked out. The goal is to incentivize teachers to stay in the classroom, rather than move toward administrative jobs with higher salaries.

DACIA TOLL:We have got to improve student outcomes. And in order to do that, we have got to attract and retain very talented people, as our teachers—as our international competitors have done. And we think a big part of that is saying, you can make this a meaningful, rewarding career.

HARI SREENIVASAN:That's something Judy Andrews finds appealing about how the model was designed.

JUDY ANDREWS:I'm not a first-year or second-year teacher, obviously. And in a traditional evaluation system, once you get to a certain point, whether or not you get any type of financial recognition is based solely on the number of years. And, after a while, that gets a little hard to take.

HARI SREENIVASAN:Retaining teachers is something charters across the country have struggle to do. In fact, a recent study by Vanderbilt University found that the odds of a charter school teacher moving to another school were 76 percent greater than a teacher at a traditional public school.

Rachel Curtis is a former educator-turned-consultant who recently analyzed Achievement First for the Aspen Institute. She says the network has had buy-in on the evaluation system from its teachers, who are all on one-year contracts without the possibility of tenure because they were consulted during the evaluation model's development.

RACHEL CURTIS, Educational Consultant: The benefit of being the size that they are is that there's been an enormous amount of teacher engagement around the design. And so I doubt that there a lot of teacher at Achievement First who feel like this is being done to them.

And that's a much more common phenomenon in a large urban district in particular, where you can't possibly have every teacher weigh in on how they think you should be going about this.

HARI SREENIVASAN:The network is paying for the teacher career pathway in part with a six million dollar federal grant. That financial reality, Rachel Curtis says, would make it difficult to implement elsewhere. But it's a model that many school districts could draw lessons from.

RACHEL CURTIS: I think that there's a huge resource allocation issue, right? And what's not clear is, is it that they just have more resources or is it that they use the resources in very different ways?

So, if you go into an Achievement First school, you will see far fewer administrators than in most traditional district schools. The choices they make about how they deploy people I think is something that we all could learn a lot from.

HARI SREENIVASAN:Achievement First says the evaluation process and teacher career pathway remain a work in progress. But the principal of Bridgeport Academy Middle School, Morgan Barth, says he's seeing significant gains in the classroom.

MORGAN BARTH: We are regularly getting results that show that we have fifth and sixth graders growing two, two-and-a-half grade levels a year and really catching up. And we're having seventh and eighth graders beat the state non-poor average.

HARI SREENIVASAN:Despite the fact that there are mixed results from national studies on whether charter schools are academically superior to their traditional counterparts, Achievement First says they're making progress in leveling the playing field between wealthy and disadvantaged students, important in a city where 40 percent of all children live below the federal poverty line.

JEFFREY BROWN: There's much more online, including a video about Bridgeport Academy's strict rules, uniforms and college expectations. Plus, tell us what you think makes a great teacher.

American Graduate is a public media initiative funded by the Corporation for Public Broadcasting.