You made good points there but I have to disgree with that one ^. There is a slight and intricate but still obvious difference between "nigger" and "nigga". Black people don't call each other niggers (except maybe if they're trying to insult one another), it's always nigga, am I right? It's always white people that call blacks niggers, and that does have a certain oppressive connotation.
I agree with the teacher that the word "nigga" isn't as loaded a word as "nigger", and it has become almost synonymous with "dude" etc. Even some (retarded) white guys call each other niggas. A while ago I was talking to this black exchange student from Canada at a school party, and at one point he called me a white boy or something to that effect (he was kidding), and in return I called him a nigga. He was a bit surprised I had the balls to call him that but he wasn't offended because he knew I'm not racist in any way. But if I'd said nigger instead, with the emphasis on the "er" part, I probably would have got my ass kicked. It's all about the context.
I don't know if that made any sense, it's kinda hard to explain with my somewhat less-than-perfect english.
But yeah, there's a difference.