"Before the 1960s, no one would have sided with the girl."
Before the 1960's she probably would have been sent home for wearing ANY T-shirt. :) I graduated in 1987 and was reminded numerous times at school to tuck my shirt in.
I once wore a button that said "No fat chicks" to school. It was confiscated and mysteriously disappeared. I don't remember suing anyone, though. ;)
At least you didn't have an Assistant Principal that would cut off your belt loops if you weren't wearing a belt through them.