Free transportation isn't an inalienable right, even in this age of everyone thinking they have the right to anything they want. This should be real easy to handle. Each of the kids involved should be brought in to visit the principal along with his or her parents, where they would be informed that Junior will no longer be allowed to ride the bus and the parents will have to supply transportation on their own. Other school districts have done this sort of thing, so they wouldn't be breaking new ground. Parents these days often refuse to discipline their kids or even teach them basic decency, but when it starts to hurt their pocketbook and cut into the money available for their own fun and games, they might take a little more notice of what those spoiled brats are doing.
Jerry