It's one of the points Joyce brings up anyway when she tells Buffy to go to college out of state, cause Faith would be there.

I really don't see how this is an example of someone commenting that Faith should be back in school. Or in any way related. Would Joyce really be encouraging a young 17-year-old to stay out of school?

Also, I thought a slayer could only be called at a certain age?

Canon is decidedly vague on that one, but see stormwreath's post further down.