The teacher made an honest mistake that ended up being offensive but he didn't have bad intentions. The kid had a right to be mad about it and report it to school officials, but he's blowing it way out of proportion, it's not like the teacher said, "I'm gonna fuckin' lynch you, you dirty nigger, right after you pick me some cotton."
Eddie Murphy and Chris Rock have been saying "nigger" in their stand-up comedy routines for years, it's all fun and games. A white teacher slips and says "nigga" just trying to identify with a student, and then all hell breaks loose in South Park.