I'm 29, I started school at a Community College recently. Sometimes I've gone as me, sometimes I've gone as my female self. Most people... just don't care. Some think it's cool, some think you're crazy. I'm generally pretty uncomfortable around strange men as it is and I'm a Computer Science major, which is a field dominated by men. I've never gotten one negative comment from them. In fact, we've all joked around and laughed and had a good time. Point is, just find some comfort being yourself and you'll do fine.
As far as the high school front goes, I've had some experience there as well. About 7 or 8 years ago, I went back to my old high school in Orlando. Went to a movie with my former drama teacher and several classmates... while wearing a skirt. Mmm.. I really wasn't passable then either. And all throughout school, I played the role of a jock and a guy's guy. No one batted an eyelash. No one really cared. I still talk to old classmates all the time (albeit mostly the girls and quite a few who had crushes on me) and everyone is just excited for me. I think you'll get a lot more support than you realize. Besides, you look fabulous, honey. I think everyone will be impressed with the woman you've become. 😀