First of all, you should defy your parents in this case. They should know that if they really loved you, they would let you live as your true gender. They are abusing you as long as they refuse to treat you like the girl you are, and suicide rates among those who are not allowed to live as their true gender are very high. Stay strong.
Let your school guidance counselor know, along with other members of the faculty and staff. Depending on your area, (especially if you are in the UK) doctors can compel your parents to allow you to be placed on puberty blockers or even take you into temporary custody so that you can begin your gender treatment. I cannot emphasize enough how important this is at your age, since you are entering puberty if not already underway. The sooner you are placed on blockers, the more successful your transition will be. In many areas, you can begin HRT in a few years. Hopefully the age will be even lower before you hit 16 so you can begin Estrogen in time for your female puberty.
It is very important that you stay strong! You are going to have to make some tough decisions very soon, and it is important that you don't back down!