It has been quite a while since I’ve been in school.
If the speaker is not at school any more, can he use the Present Perfect after "since"?
If I were a native speaker of English, I would never shut up. :-)
That's fine, although in BrE I would change "in school" to "at school".
It's a long time since I was at school.
It's been a long time since I've been at school.
The former is probably more natural but the second isn't incorrect.