I would definitely go more on safety and what gender most strangers read you as than on any existing local laws. People are not likely to object if nothing seems amiss.
I understand that isn't easy, and most of us spend a significant amount of time not being reliably read as one gender or the other. I am not you, but if I were, I would try to speak privately with administrators at your school about what you are doing. If they resist allowing you to use the restroom you prefer, then contact local lgbt organizations for advice and don't be afraid to stand up for what you feel you need. In most places, there aren't a lot of official rules about this, and you can help shape any that come into being.