City officials often defend their strategy of replacing large, struggling high schools with smaller ones by arguing that it’s the only proven way to boost student achievement in those schools.

Today, a top official in the office that supports schools as they phase out said that the reasons for why that strategy works remain a mystery.

At a City Council hearing today called to discuss how the Department of Education monitors students in schools as they phase out, officials argued that as schools closing shrink by a grade each year, students receive more individualized support from remaining staff members.

Josh Thomases, the Deputy Chief Academic Officer of the DOE’s school support division, cited a 2005 New York Times story that described the final years of Morris High School in the Bronx, when students reported receiving more individualized attention as the school shrank. The piece recounts an incident in which a student left school one day, forgetting that she had a second Regents exam to take that afternoon. An attendance teacher was dispatched to pick up the student as she got off the bus near her home.

City Councilman Lew Fidler asked Thomases a question that is often asked by school closure opponents. “Why do you need to close the schools for that support to happen?” Fidler asked.

“The truth is, I wouldn’t say that I fully know,” Thomases responded.

Many critics of the city’s closure policy argue that struggling schools fail precisely because those kinds of supports were cut, or were never given to the schools after they began to serve increasingly large populations of high-needs students.

Thomases responded that regardless of support, there’s a limit to the number of high-needs students a large high school can serve well. Most schools of any size can boost the achievement of a certain number of struggling students, he said. But when a large high school enrolls a much larger number of under-prepared students, it begins to struggle to raise their performance quickly.

“It is simply too high a bar to move quickly,” Thomases said.