Solvency = Greediness?

The AP has a piece today about an aggregate audit that was performed on the state's charter schools. The study finds that 46 of 57 charter schools are running surpluses, which to me seems like a positive thing. Of course, charter opponents immediately argued that charter schools are being irresponsible.

"Glenn Koocher, executive director of the Massachusetts Association of School Committees, said the report shows charter schools are "stockpiling large reserves" when other public schools are struggling to make ends meet.

"It cries out for reform of the charter school funding system," Koocher said. "If they've got money to give back, why don't they give it back to the people they took it from."

Seems to me that charter schools need to be financially solvent to survive. It's not only good budgetary policy, but it means that financial mismangement can't be used as a reason to shut them down.

How is it that charter schools are running surpluses when everyone else is running deficits? Is it because of lower teacher costs (non-union salaries, benefits, etc.)? Is it because of fundraising? I'd be interested to see what percentage of the aggregate surplus ($91.5M) is due to fundraising, and what amount comes from other cost savings.