The most important thing is don't show your fear. I graduated hs last year and it was only fun to mess with the kids who looked intimidated and just kinda play on their own ideas. We didn't do anything and weren't going to, but if they looked intimidated we played on their imaginations pretty well...
But don't act cocky or arrogant or like you actually mean something. If you do that then it's annoying and grade 11s and 12s will put you in your place. The annoying cocky grade 8s and 9s are the ones who get beat up, duct taped to walls, shoved in lockers, tripped in hallways, cut in front of in lines, etc.
So just don't get in their way and don't act scared. They aren't gods or anything so don't be afraid to talk to them if there's an opportunity.