Actually, according to my English teacher, ain't is a real word - in one circumstance.

I am not
I ain't

It's a contraction for "am not", and because "am not" is used only after "I", it is incorrect unless it has an "I" in front of is. Thus, "he ain't, she ain't, we ain't" are all incorrect.

But I ain't is not.

That said...it doesn't exactly sound educated to use it, so I wouldn't recommend it.