She feels she can't practically ignore out of hours requests and she doesn't want to challenge the practice for fear of rocking the boat.
Unions are really powerful in education - it's one of the only benefits of the sector I can think of. Hopefully your wife has a union rep they can teach out to on behalf of all the staff if not just herself? Teaching assistant salaries definitely don't to high enough to compensate for anything outside of school hours!
It's not that they expect everyone else to be working these odd hours at all, just that it's the only time they themselves can catch up on work.
One colleague who is a manager even has a note to this effect in his email footer, explaining he doesn't expect others to action or respond to messages outside of normal working hours.