[equark]

You underestimate researchers. Recent research on charter schools uses randomized assignment, induced by oversubscribed schools. They often find large positive effects:

http://papers.ssrn.com/sol3/papers.cfm?abstract_id=1517008

http://papers.ssrn.com/sol3/papers.cfm?abstract_id=1550609

Not surprisingly, the results depend on the school type and some charter schools don't work or only help specific groups:

http://www.mathematica-mpr.com/newsroom/releases/2010/Charte...