Groped on a Field Trip (15/f)

I’m 15 years old and I was on the sky train coming back with my class from a field trip. On the way back to school I noticed a guy a few years older than me staring at me. He came up close behind be and stood there for a couple stops. I already felt a bit strange but didn’t think much of it I kept on talking to my friends. But then when we reached a stop he was getting off the skytrain but right as he was getting off he squeezed my ass. I felt that it was quiet disturbing considering the fact that I’m underage and its just plain rude. I didn’t get a chance to see exactly what he looks like nor can I be certain it was the guy who was staring at me because I was facing away when it happened.