Who said anything about the kids making that decision? Certainly their parents must make the decision whether they should be in school. However, if we're not forcing everyone to be in school (read the Gatto article), schools are free to expel those who disrupt the learning process of other students. If parents are too apathetic to make their children go to school, it is not the responsibility of the state to take on this function. In that light, there is no need to even consider an age under which it should be compulsory to attend. Compulsory attendance is a protection mechanism meant to preserve the institution, not to benefit the participants.
"(And you've called me an elitist liberal before. It's been a while ago.)"
Yes I did. It was because you were making elitist liberal comments. However, you referred to one of my statements as elitist, which it clearly was not, hence my suggestion that you look up the word.