Is it forbidden to eat biscuits that are surplus to the students' needs and are distributed to teachers to avoid the routine of returning them to storage or spoilage, on the grounds that they are not rightfully the teachers' due?

The default is to return any leftover food to the school, to the party responsible for it. It is not permissible for teachers or workers to take it unless there is explicit or implicit permission. The mere routine of returning it does not make it permissible to take it. However, if they know that the leftovers will be thrown in the trash or will spoil, then it is permissible for them to take it; in fact, benefiting from it and distributing it would be better.