You pull it on hastily, and your own body starts to change. For one thing, you shrink a little so it fits you perfectly. Your eyes turn a deep brown and your hair becomes jet black, while your skin turns a similar but slightly darker complexion than hers. You feel like you've lost a few years, too.
When you look at your girlfriend, you realize you're both 17 and classmates. The room suddenly shimmers, and you realize it's a classroom at dusk. You walk up to her, cup her cheeks and kiss her passionately. She kisses back and, playing along, smiles, "babe, we can't. What if we get caught?"
"No one will catch us," you reply confidently, and carefully push her to the floor. You both breathe heavily as you grind on her, and while she reaches to undo your belt, you pull up her skirt and unbutton her shirt. Both of you are aching with hormonal lust, which only intensifies when you reach behind her back to pull off her bra and grab at her now exposed breasts, and when she pulls your trousers and boxers off in a fluid motion and grabs at your cock.
A moment later, you've pulled her panties off, thrown them aside and shoved your dick inside her waiting pussy eagerly. Thrusting hard and kissing her with tongue, as well as sometimes kissing and sucking her neck and boobs, you feel amazing, especially with the horny teen side of you that the change has brought out brimming over with pleasure at the thought of getting to fuck someone and knowing you'll cum inside her.
You're sort of aware of some weird changes, particularly that you hear her murmurs change from 'yeah' to 'hei' and understand it instinctively, but both of you are so wrapped up in the pleasure you hardly care that much. Before long, you came hard and filled her with your seed eagerly, and you clung to each other eagerly in the afterglow, kissing occasionally.
While you were recovering, things started to change...