You really enjoy being such a tight pair of panties, and you're impressed with yourself for coming up with such a great spur-of-the-moment decision. That's why you decide to ratchet it up a bit and go even smaller. You imagine yourself turning one size smaller, and now you're grabbing her ass and massaging her pussy even tighter. It's as close to Kristen as you can imagine and everything you'd hoped. Plus, you're getting a real kick out of putting Kristen in such a difficult position. At this point, you figure wearing her panties is pretty uncomfortable for her, and she probably really wishes she could get out of class to figure out what the hell's going on. Unfortunately for her, she just ran off to the bathroom for your previous little trick, so you're sure she feels stuck in class until it ends. Hilarious. For now though, you're just going to enjoy being ever so closer to her: every tiny shift makes an enormous difference at your perspective. Even her breathing makes you rub against her pussy and her hips, almost immeasurably. She's trying hard not to move, as you're digging into her in several places around her waist and legs, probably causing a good amount of annoyance and even pain, but there's not much she can do.
You love this experience, but you know it's probably going to end soon. Kristen is not going to tolerate wearing panties this uncomfortable for a second longer than she has to.