If someone is looking after my child, I have their number. I block calls from unknown numbers adn and some prefixes, and silence calls from numbers not in my contacts. This is pretty normal I think (because of spam).

I've only once had the problem that someone from the school used their personal mobile to call me and didn't get through, but I was already calling to let them know I'd be late.

There's no reason for parents to be able to contact you socially unless you invite it, but surely you should contact them from a school/shared number?

I'm suggesting this not for the sake of others, but for your sake. Given that people will block unknown numbers, I would think using a known number makes your life easier.