Comment by Symbiote

The school's own teachers would cover for an injured colleague in an emergency like this. The lesson timetable will be written to ensure there's always some teachers free.

Given more notice, the school phones supply teachers (or an agency for them). Some of these teachers will expect to be ready to get to a school with very little notice.

In the worst case, where the cover teacher has not had chance to discuss what lesson is needed, they're going to say "let's go through the last chapter in the textbook" rather than "I give up, fiddle on your phones".