Last summer I read Sowel's book on Charter schools, he has a ton of data in there, quite interesting. My take from it was this. Basically discipline work ;) unlike normal school successful charters can and do kick violent kids out. In many cases if you get rid of the worst 10% rest can do very well. That and being able to fire useless teachers not he spot.

I think you are right that with school where kids can not get kicked out and discipline is not enforced it is basically all based on people going there. Once school can actually act as a school things change quite a bit