THat's not cool that they laughed about it, because yes, it was very much sexual harrassment and not right. Don't let their laughing invalidate your feelings. However it made you feel is what matters, not how she may have meant it. No matter what her age, grabbing your butt is very inappropriate and a boundary violation.

I agree that you should never be in the room alone with this person again. Or should just never be in the room with this person period.

Seesaw