Students ... be late for school.

The sentence expresses a prohibition or a strong recommendation against being late. The modal verb 'mustn't' is used to indicate that something is not allowed or is forbidden. Therefore, 'mustn't' is the correct choice to convey that students are not permitted to be late for school. The other options, 'have to', 'must', and 'don't have to', do not fit the context of prohibition or strong negative obligation.

The correct option is: mustn't