how to know if you have been hacked on facebook?

[Own Experience]

well, i know this is kind of strange, but i heard that my school has access to all of our facebook accounts and that really freaked me out. apparently they got themselves a hacker or something and they have seen all of our messages, posts, etc.
I don't use facebook anymore tbh but still! like, isn't that illegal?!?
a couple of years ago apparently there was this whole scheme to go on strike bc some people didn't like the new rules or something, and it was all through fb, so maybe this is why they are doing this now.
but like what i am asking is, is there any way of knowing if someone else has accessed my account?? and is what i heard even possible? has it happened before? is it more common than i think? this really freaked me out...what do i dooooooo
thanks