I think, actually, they are right, their juristiction ends at the school gates, and its not happening in the grounds.
But I would have thought, if they now know who the girls are, they could have a word with them, I mean, how many times have you picked up a newspaper, and a child has self harmed, and worse, because of bullying?.
Go back to the school, if you can, try not to be confrontational, and impress on them, the possible consequences, if its not stopped.
Have you spoken to the Bus driver?.