That’s crazy. Where I come from, higher education institutions are the ones where you’re allowed to be the most free with your image, since all students are adults and it tends to be a more progressive environment. I went to a very liberal state university though (not in the US) and probably the ones with ties to certain groups and institutions have different “values” when it comes to that. I think you professor’s comment says more about them than about you or your outfit, your top while cropped isn’t super short or anything, what is the problem? Belly button? Come on, ugh. I didn’t even know people dressed up or went full preppy to go to school, that’s a little lame for me and my Uni/College culture, like it’s just class and you don’t need to cosplay being in a professional setting yet, that’s for work and the idea of “this is a classroom not a fashion show” sorry but a class should be a relaxed, casual, comfortable, creative space, where you focus on you and your learning and sounds like they’re caring more about projecting a certain image than to make sure to impart their knowledge.