Really excellent angle, and some nice attention to detail on the knuckles underneath the panties where they have become... moist. Interesting shirt though; it seems as though it is either hiked up, or simply very small.

Why the hell was this marked "Safe"?

I think the shirt is both small and hiked up. She probably got a normal sized shirt for girls her age, but her breasts are so big that they lift the shirt up a lot. Looking at the light from the window, it's still noon class time, but she doesn't care as she masturbates. She's not even trying to do it discreetly. She just spread her legs as wide as possible and put her hand inside her panties to masturbate. Her nipples are so erect that they are visible from her shirt. This should all look obvious from the front, but I guess she doesn't care as she turned too slutty and wants to cum during classtime.