We did the exact same, with water ballons and whatever "weapons" we could carry. I would also moon the bus just after I got dropped off at home as they drove off.Growing up, our bus driver would allow us to have a water gun fight on he last day of school. We had to wait until we were out of view of anyone else because he was definitely not supposed to allow that to happen, but once we got about a mile from the school, it was carnage until you got off.
I looked forward to it every year and it was one of my favorite childhood memories. RIP Frankie