With my OCs, there would be a lot of chaos. Especially in regards to ones who can cast spells such as Barbara Autruche or Tihun Habtamu. Just imagine if they got angry (Tihun, despite normally being a nice girl, sometimes puts curses on people who wrong her). Not to mention a lot of my OCs have outrageous fashion sense. Just look at my dA art of Narcisse Paon and Ari Lutin and imagine them walking around the hallways of your school. At least half of my OCs wear clothing you'd probably never see real American teenagers wearing on a daily basis. (Well, most of my OCs are either princes or princesses, and many of their clothing styles are closer to European styles than American ones...)
I think it would depend on the OC. Orhan Sessiz has clothing normal enough to pass for a real-world teen. Maxime Autruche, on the other hand, would definitely stick out like a sore thumb with his ostrich-feathered pickelhaube and Fauntleroy suit.
As for the canon characters, they'd probably have an even harder time fitting in since some of them have things that you don't see in real-life (such as Cerise Hood being part wolf), plus their clothing is quite elaborate. The guys might have it a little easier - Daring could pass for a normal teenage boy if you took off his crown. Dexter and Sparrow could also pass for real teenage boys.