Girls can sit a certain way, with their leg bent and near their crotch, that allows them to rub their cooch with their foot while sitting in a chair, and it doesn't arouse suspicion.
It would probably be alot easier if the teacher was ....preoccupied.
Spoiler:
Not to mention, all girls have to do is stick a toy down there and click a button - as long as it isn't too noisy and they can keep their moans to themselves, they're fine.
Well, I'm pretty sure I did it when I was little, like before I knew what it was or ejaculated or anything.
But now, it wouldn't be possible without it getting noticed, I don't think.
dude got caught fappin in one of my classes when i was in H.S., was the most hilarious shit ever, he turned redder than an apple and ran out the class without putting it away. needless to say he never came back XD