Sudbury Valley does not self-select students. It is a private school, but if you can afford the relatively low tuition ($9k a year) and want to go there you can. There is no selection process other than making sure the family is on board with the educational philosophy.
I think you've just described what 'self-selection' is: the school is not selective, but the students who choose to attend the school may not be representative of the general population
I agree. The students at Sudbury Valley and the families are not representative. That said, there is no application process outside of making sure you want to be there and your family is on board. They are not filtering out lower-performing students at all.
But it is certainly possible that this model would fail miserably when applied more generally. I don't think that can be the null hypothesis, though...
