Most people here are assuming one of two things (or a combination thereof):
1) The parents are to blame for the child going off the derech
or
2) The school is responsible for the child going off the derech
Obviously, there are many variables and it differs from person to person. Nevertheless, between those two possibilities, I would venture to say that the parents tend to bear more responsibility for the problem than the school. The parents are responsible for their child 24/7. The parents are in charge and with their child more hours per year than the school.
That being said, what everyone seems to be missing is who is truly the most responsible for the person going off the derech. And that, of course, is the person himself who went OTD. Yes, he is most responsible for his bad actions. Yes, he is the one who will face the direst consequences for his terrible decisions. He will face consequences down here; and, most importantly, he will face the music upstairs.