No - the offensiveness is not in the race but in the mockery. It's just trashy and had no business in algebra (or any other) class.
That said, the mother's an idiot. "Who's protecting us?" From WHAT - getting your feelie-weelies hurt? It's a mean old world, sister, get used to it. Not sending him to school until it's 'safe'? Again, from WHAT?
Personally I think the teacher was mocking ignorance and I'm just not that offended by it.