You've got a thread from her sweater

A single strand of wool from her black sweater is tied around a desk leg at the far end of the room. Meaning that as Selma tiptoes towards the door, the garment is slowly beginning to unravel. From the bottom up.

She doesn't notice at first, not until she's halfway across the room. By that time, people are starting to point and stare. It's not until she feels the breeze on her legs that she looks down and sees her sweater now comes only halfway down her hips, and her white cotton panties are on display. The curve of her full bottom peeps out from beneath the black woollen hem, and her plump thighs are bare right up to her mound of Venus, which is clearly defined beneath her underwear.

"Hey, look!" the elf shouts. "That girl's got no pants on!"

Everyone starts to laugh as Selma squeals, cups her hands between her legs and makes a mad dash for the exit. Of course, that only accelerates the unravelling. By the time she's out the door the hem of the sweater is at her navel, and her panties are revealed for everyone to see. By the time she's made it down the stairs it's beginning to expose her white bra. By the time she's on the campus green it's unravelled completely, leaving her with nothing but a little black wool collar to complement her white bra, her white panties, her black shoes and her black thigh-high stockings.

Selma is now running across the crowded campus green in just her underwear, squealing her little head off, cupping her big bouncing breasts with one arm and trying to hide her panties with the other. You call that a good start.