Steal Selma's skirt

This is going to require some finesse. Though not too much.

Raider turns himself invisible, then creeps across the floor until he's underneath Selma's desk. He uses a pair of scissors and a little bit of magic to very slowly snip away Selma's skirt, piece by piece, starting with the hem and working his way up until he's able to sever the waistband. You can see her white panties peeping out from between her legs as he gently tugs the skirt out from under her, applying a bit more magic to make absolutely sure she won't notice.

He creeps back to you, and hands you the shredded skirt. "Let's see how long it takes her to notice," he whispers.

It's about fifteen minutes, and you're starting to get very bored when Selma finally looks down and realises, with a muffled squeal, that she's been surreptitiously depantsed. Her eyes dart back and forth, trying to work out who could possibly have done this, as she tugs the sweater down over her hips. You're very well hidden, of course.

She stands up, a blush springing to her pretty dimpled cheeks as she scans the room. As long as she's pulling it, the sweater comes down to just past the hem of her panties, revealing a lot of thigh but making it look like she's not quite in her underwear. Carefully, she puts the book she was reading back in the stacks.

Then she starts walking towards the stairs. It looks like she's decided to make the walk back to her dorm room and get a change of clothes. Of course, that'll mean going through the crowded study areas down below, and across the busy campus green. Should be a lot of fun for a shy nerdy girl with no pants.