Is it permissible for me to have bought something useful for the school instead of the supplies that were damaged, or must I return the value of what was damaged?

If the items are simple and there is no demand to return their like to the school, then nothing is incumbent upon you. However, if returning them to the school is the known practice, then it is obligatory to return them or their like if they are damaged. Their value should not be returned unless returning their like is impossible. It is advisable to ask the school officials; if they inform you that their return is not required or they accept a substitute, then nothing is incumbent upon you.