Yeah tell that to my teaching assistant wife who works at a Lincolnshire village primary school and is now 'always on' to the whims of the go getting headmistress 24/7.
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.
I empathize after two years in UK education. The working culture is almost cult like and self reinforcing - the Peter principle in action turns the best teachers at reinforcing the status quo into headteachers.
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!
As a counterpoint, I'm not in teaching but many of my colleagues workdays are heavily disrupted by childcare, so they compensate by working odd hours, sending emails or slack messages at 5am or 1-2am etc.
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.
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.