You're going to need to search everywhere, preferably when the room is empty so you can hear. Check cupboards, closets, any thing with a small opening. Listen for any hissing or huffing that might signal where she is. If you can, get a space heater set up in the room to heat it to 73* or so. It's dangerous for her to be in the room with that temperature, especially since it's usually even colder at the floor & she could be in cooler areas as well, wherever she's hiding.
Any chance you or someone you know would have a live trap used for wild animals? Or could buy one? One big enough for a hedgehog wouldn't cost too much, and they're often sold at outdoor & home improvement stores. If you can't find your hedgehog today, I would get a live trap to set up tonight or tomorrow. Put food only in the trap, and a heating pad under it (but with a blanket between the pad & cage). Hopefully if hedgie is warm enough to come out looking for food, he'll end up in the trap.
It's essential to find him as quickly as you can. If there's any openings at all in the walls (vents, a big enough hole, etc.) he could end up in the walls and be completely lost. In addition to the heating concerns, it's dangerous for hedgehogs to go longer than a day or two without eating. You definitely need to find him before Christmas break - if you don't by then, I would assume he's probably dead.
Also if, hopefully when you do find him, be prepared that he may be attempting hibernation or have done so & managed to come out of it himself. It's very possible he might need a vet visit in case he has a respiratory infection now, or other injuries from escaping & being out in the room. You'll need to make sure their cage is 100% escape proof as well.
Although honestly, I'd recommend taking them home again and keeping them there. Hedgehogs aren't great classroom pets and they don't do well in that kind of setting for the most part. They shouldn't be left unattended for a full weekend either. They would be better off at your house where they can receive attention every night at a time that's more comfortable for them and make sure they're being attended to. A visit to the classroom once in a while is fine if it doesn't stress them too much, but it's not really appropriate for them to stay there longer.
