Just because a dog is protective doesn't mean its vicious. Also "Pit Bull" isn't a breed of dog its a term used to describe many different dog breeds that look like American Pit Bull Terrier/American Staffordshire Terrier, once again another stereotype. So technically this dog could be a mix of about 16 different breeds so saying that "Pit Bulls" are vicious aggressive dogs is one of the biggest stereotypes I've heard in awhile.
American Pit Bull Terriers believe it or not are very friendly to EVERYONE including strangers and other animals if raised/treated right. They can turn out just like any other dog out there and really this story means nothing because they don't even know what kind of dog is was, seeing as they labeled it a "Pit Bull". The only thing I think is bad about this is the fact that the kids were teasing the dog, and the kid who doesn't tease it gets hurt, I think the families of the kids who were teasing it should pay any bills/costs along with going to counseling. The dog is NOT to blame.