I heard someone mention this site saying that if some online schooling services caught you using it, you could get expelled. Is this true?
If they know about jiskha, Probably
as long as you only use it to get some help, I see no reason to disallow it. That's no different from studying a book or video. The goal is, I hope, to learn to solve problems or answer questions.
If you use it to cheat, though, that's another matter.