It is actually against the law in PA for an adult to enter a school bus (other than driver or teacher). The schools will do nothing to protect children on buses. They can put children off the bus for a short period of time, but not keep them off.
The only way to fight back is to go directly to the parents of the children, the schools will do NOTHING. My experience was exactly that; after complaining about my son being abused by another student for a year the school had done nothing. They assured me it was being taken care of. The first day of the next school year it began again. I called the mother of the student on the phone and asked for her to help both our children arrive to school safely. She had NO IDEA it went on for a year, they had never contacted her. It never happened again. Had that not resolved it, assault charges were next.
Wow, 155 posts, and finally someone (you) posts the correct way to handle the situation. Kudos and good on you.