I joined an all-girls school, and have regretted it ever since. I've been at one for four years now, and understand how every little thing can get under your skin and make you feel down (Changing rooms, the whole class being referred to as 'ladies' or 'girls', being called 'she' all the time or 'good girl' for good grades, bathrooms, etc.). Not being able to tell your friends is the hardest part, from my experience. I plan on telling my closest friends on the last day of school, just because I'm afraid our friendship will fall apart.
The best you can do is to manage your time at school based on your personality and own experiences, for example some people might hate spending all their spare time alone working but I do it every lunch time because I'd rather go to another class than be surrounded by girls who don't understand me (I also have some awesome male teachers I get alone really well with who will let me in their rooms any day so then there's that...)
Things which waste time are the best, if you don't feel ready to come out at all. I would advise spending extra time doing things which you love to make something you can look forward to until you can leave/would be able to convey how you feel to staff. I think leaving is the best option, even if you are leaving close friends and your girlfriend behind. I'd just think that you're in school anyway, you have to spend most time working and not enjoying yourself in the least (especially given your circumstances). Moving gives you a chance to be open from the start and be treated like you want to be without disagreement because it would be 'weird to have a guy in a girl's school' and be ignored, I would think you would also enjoy your time outside of school more since you wouldn't have to worry so much about the next time you have school.
If you aren't out to your parents either, I think it would be best if you were- even though it is probably the hardest thing you can do.
I hope you can get all of this sorted out in the best possible way for you.
