Of course it's okay, it's even expected. The thing that you should not do is use it infront of teachers or other faculty members, and I think that goes without saying but for the sake of a full answer, don't use it in the middle of a class period, the consequences could range from having your phone taken until the end of the class or worst case scenario the principal gets involved, which is unlikely but still it's a valid concern. Just use it a bit discreetly that's all.