During the time he was supposed to be in school, the school is liable for anything he does while in their custody. In some cases, they are liable even after he leaves their custody.
If there couldn't have been direct two-way contact with the parents, the student should have been required to be there.
The contact numbers are no excuse. Most schools verify the contact numbers when they are originally submitted, again periodically, and when they change. There should have been primary and secondary contacts on file. This is a screw-up on the school administration's part, pure and simple.