There are now 136 charter schools in the city, and dozens more are opening every year, but now a report by the New York Charter School Center is trying to pinpoint what works with charters and what doesn't. NY1's Education reporter Lindsey Christ filed the following report.

Publicly financed but privately run, charter schools are a lightning rod for controversy.

Supporters argue they often outperform local schools, giving parents better options. Critics say that is because they attract better students, leaving local schools overwhelmed with more needy kids than before.

Now a new report helps shed a bit more light on who charter schools are serving and how they compare to the schools down the street.

The report comes from the New York Charter School Center, a nonprofit that supports charter development, but it is frank about charters' weaknesses, as well as their strengths.

"It's the first step in trying to be as transparent as we possibly can and hopefully have a productive conversation on how we arrive at a day when every public school, charter or district, works for every child," said James Merriman of the New York Charter School Center.

Charters have higher test score averages and attendance rates but do not serve as many high-needs students as the average school in their districts: 72 percent of charters serve fewer special needs students.

The report also finds 80 percent of charters serve fewer low-income students eligible for free lunch and 96 percent of charters serve fewer English Language Learners.

Charter schools also have much higher rates of teacher and principal turnover: 30 percent of charter school teachers left their position last year. In public schools, it was 13 percent.

It is unclear why. Do charters, most of which aren't unionized, do a better job removing bad teachers? Or are they difficult places to work?

"This report is a first report and it is a starting point for research," said Merriman.

Another area that needs more research is whether charters push struggling students out of their schools.

The report reveals that charter middle schools do lose about 6 percent from each grade annually, while in district schools the number in each grade increases as students move through middle school.

There are several possible explanations for those numbers, but the answers won't be found in this first report.