I mostly agree with QO, but it depends entirely on the teacher and the student. There is absolutely nothing wrong with cracking the whip on an unmotivated child so that you teach them there are consequences to their lack of motivation when it doesn't matter as much. Take a really motivated kid, and they'd flourish at Summerhill...but they'd probably flourish anywhere.
Personally, I'd never ship my kid off to a boarding school. What's the point of having kids if they don't live with you?