If the problem lies within the school, it is your responsibility as a parent to protect your kids.
I would report the problem to the school. They could check each child at the school and notify parents if their child has lice as well. It would be almost impossible to prevent your children from catching lice if a lot of other kids at their school are infected as well.
Basically you have to wash EVERYTHING.
I would treat the kids with Rid every week if this is the case, at least until the school is able to check each child for lice.
I don't know what else to say.