Note: Daniel Lautzenheiser, program manager in education policy studies at the American Enterprise Institute, is guest posting this week.

If there’s ever a tangible physical reminder about the differences in education quality in a particular locale, it is found on the floors of a school building on West 134th Street in Harlem. Literally on the floors. There, a line of tape runs down the hallways. On one side is Democracy Prep Charter School, which in 2010 was the best middle school in New York City. On the other side, until recently, was the Academy of Collaborative Education (ACE), which that year happened to be the worst middle school.

I’d heard about co-location before but had yet to actually see it until a visit a couple weeks ago. Limited physical space in New York and other urban areas means multiple schools share a single building. It’s a remarkably complex and contentious issue, bundling up all at once questions of school funding, budget and cost constraints, space and other resource restrictions, equitable funding, and disparities in student achievement.

As such, co-location is often seen as a symbol of what is wrong in public education. Opponents paint a picture of crowded hallways and unsafe teaching conditions, of educational inequality, of one school siphoning off limited public dollars at the expense of another. And likely there is an element of truth to some of this. In the particular instance of Democracy Prep and ACE, however, co-location offers a tale not of what’s wrong about the state of public education, but about what’s right--about what happens when accountability works, when consistently failing schools are shut down and good schools are allowed to expand.

The story is this. The first campus of Democracy Prep (the network now has seven schools) opened in 2006--the exact same time the New York City Department of Education (DOE) opened ACE. In hindsight, the case offers about as close to a scientific experiment as is possible in K-12 education: two brand new schools, both teaching about 100-125 6th grade students, located in the same building, with the same kinds of students, all from Harlem.

From day one, the two middle schools moved in literally opposite directions in terms of student performance (as measured by the DOE chancellor’s annual Progress Report) and parent and student satisfaction (as measured by NYC School Surveys). A short three years in, in 2009, ACE received a “D” score on the chancellor’s Progress Report and was ranked the worst middle school in central Harlem, while Democracy Prep received an “A” and was ranked the best.

That year, DOE put ACE on a school closure list with 18 other failing schools. ACE, designed to provide Harlem students with rigorous math and technology programs and matriculation into top New York high schools, had moved in a consistently negative direction in terms of student learning and safety. But the United Federation of Teachers (UFT), the city’s largest teachers union, joined with the NAACP and other groups to sue to keep ACE open, arguing that DOE had “failed to indicate the ramifications of the school closings.”

Initially, the court ruled in favor of the UFT, and ACE was granted a lifeline. It was short-lived. By the end of the 2009-2010 academic year, ACE had become the worst middle school, not only in Harlem but in all of New York City, while Democracy Prep had improved to the top middle school--again, not just in Harlem, but in the entire city. The charts below from the chancellor’s Progress Reports and School Surveys succinctly show these trends.

Chancellor’s Progress Reports:

*In 2010-2011, ACE’s enrollment dropped to 129 while Democracy Prep Charter increased to 340

2009 School Survey of Parents, Teachers, and Students:

It was at this point that the disparities began to attract the attention of local news. According to a New York Post article at the time, “Only 3.4 percent of [ACE’s] 195 students--about six kids--read at or above grade level...That’s the rock bottom of the 41 ‘peer group’ middle schools. It comes in next to last in math, with 9.1 percent at grade level... It also flunks safety. ACE made the state’s list of 16 ‘persistently dangerous schools,’ one of 12 in the city.” In July 2011, the courts reversed their decision and allowed 22 persistently failing schools to be shut down, including ACE.

At the same time, Democracy Prep has expanded, opening a second middle school, Democracy Prep Harlem, in 2010 with 109 students. DOE located Democracy Prep Harlem in the exact same space that ACE once occupied, meaning it now serves similar students, in the same community, in the same building but with massively different results. (And to be clear that this isn’t a case of DOE blindly giving preference to charter schools over zoned neighborhood schools, there is another middle school--PS 92, a sort of average “B"/"C” rated school--located with Democracy Prep in the same building on West 134th street. In other words, DOE concentrated their accountability efforts on the truly persistently failing schools.)

Two takeaways:

First, in a local setting, this robust expansion over the past six years has contributed significantly to a groundswell of high-performing charter schools in Harlem. In the case of Democracy Prep, due to continued growth and a New York state law that requires charter schools to first fill their seats with in-district applicants before handing admission to out of district applicants, all students in Harlem who want a kindergarten or sixth grade seat at a Democracy Prep campus will have one. What’s more, Gotham Schools reports, “half of the 1,700 District 5 sixth-grade students will attend charters” in 2012. This makes Harlem an incredible case study for the proliferation of top charter schools. It will be fascinating to see how this plays out.

Second, more broadly, is this story serves as an example of successful accountability. There’s no reason to be flippant about this. One school, for four years in a row, did an increasingly poor job of educating its students, who are now living with those consequences. And even shutting down ACE had ramifications for the teachers and staff who worked there and for the students who were sent to other schools. The process is undoubtedly hard. But when accountability works as intended, consistently low-performing schools are identified; a district is bold enough to take on and win a lawsuit to shut those schools down; better schools are allowed to expand to fill the void; and students, ultimately, are moved from low- to high-performing schools.

--Daniel Lautzenheiser