The only bad things I experienced (and I was on buses in elementary as well as to HS) was just plain “bullying”. There were always kids picking on others - including me (not in HS, though; HS was a peach for me) - and sometimes getting a bit physical (usually not until off the bus, though).
I don’t ever recall nasty sex stuff, I have to admit.
I did see bullying and drug use though.