This is a college, after all, and one of the leading colleges in its fields of specialty at that. They don't let in just anyone, you have to display some qualifications and be accepted in. Fortunately for you, transformation was what you showed skill in. Not to Robbie's level, but hey, he's had a head start.
So, as some may have heard, shapeshifting spells can be hard to come by in some circles for their difficulty. Things tend to want to go on being what they were. Your body wanted to be a human (even if your mind disagreed sometimes). You let the transformation to a horse happen because it caught you off guard (and you hadn't had any practice guarding against this kind of thing), but you paid attention to the energy, the feel of the magic and its push against your body's resistance, and as soon as you have your wits about you and Robbie walking away you take hold of the energy (it's still there, as many of the morphing spells lasted for weeks, months or years, instead of days or hours, but did wear off eventually if not refreshed) and redirect it. One trick you learned about magic in your studies was that the energies within yourself are the easiest to control, once you know where to reach for them.
Your legs lengthen, and your fingers which spit back into five while retaining the hoof-like nails, and you slowly stand up, like after being knocked down in a fight. That's how you feel, really; being forced into a form you weren't prepared for or were resisting isn't pleasant. You look up carefully before reaching your current full hieght, and keep your joints bent to keep from hitting the roof. You keep a lot of the horseness, at least for the moment, to hold the energy.
One problem with shapeshifting spells is it can be tricky to get the energy into yourself (instead of someone else) to begin with. Changing completely back human would make it hard to hold onto the energy for anything useful, especially while not concentrating on it (like while studying another subject or sleeping). But if you just stay at least halfway or near it, you should be able to bend it back and forth for as long as the spell would have lasted, and probably even refresh or enhance it with less trouble than shapeshifting yourself from scratch. Really, Robbie's given you a gift.
Another good thing about staying part horse is the sheath, considering your clothes are shredded. You're more decent in this form than if you went full human, at least so long as your physical arrousal stays down. You could try transforming the shreds into something to wear... It might not even be hard, but you've never tried something like it before. You reach down into the tatters and pick out everything valuable from your old pockets. Then you think of something you have done before, making fabric lengthen endlessly like one of those entertainer magicians but for real, and do that to make yourself a sling out of some of the shreds so you don't have to carry all your stuff in your hands.
You open the gate just to see it's not locked, only latched so a horse couldn't get out, then stop to decide whether it's worth the trouble to make yourself more to wear (worse case scenario you could wrap up like a mummy), or just head to the registrar to deal with your schedule like this (either you need to get it back as it was or find out what it is now; maybe you like polo). And find your real room and figure out what to do about Robbie after that. You might even be able to send the spell back at him, but you'd really better have something on before doing that.