There are three answers that come to mind:

1. As Keith mentioned in his comment, security. You don't want a random guy just appearing in a dormitory at midnight. It would be nearly impossible to police comings and goings, and there would be no guarantee that the students would be safe.
2. Apparating could encourage truancy. The students could disappear at will and leave overnight. While many students do sneak out or around in the books, attempts are made to apprehend them, sometimes successfully. This couldn't happen if apparating were a possibilty.
3. Apparating is, in and of itself, dangerous. As in, do it wrong, and you could die. This is not the kind of activity that you want inexperienced students experimenting with. There would be bits of children popping up all over the place, or they'd get stuck in walls. The mess would be obscene.

There are three answers that come to mind:

1. As Keith mentioned in his comment, security. You don't want a random guy just appearing in a dormitory at midnight. It would be nearly impossible to police comings and goings, and there would be no guarantee that the students would be safe.
2. Apparating could encourage truancy. The students could disappear at will and leave overnight. While many students do sneak out or around in the books, attempts are made to apprehend them, sometimes successfully. This couldn't happen if apparating were a possibilty.
3. Apparating is, in and of itself, dangerous. As in, do it wrong, and you could die. This is not the kind of activity that you want inexperienced students experimenting with. There would be bits of children popping up all over the place, or they'd get stuck in walls. The mess would be obscene.