First of all, I am NOT the best person for this, as I have neither transphobic parents (god BLESS) or a desire to medically transition (also god bless). I’m very, very sorry that life has put you in this situation, it sucks.
If you only want to socially transition (or medically transition), present your parents with the science that trans kids are happier and more mentally healthy when they’re allowed to transition. Try to reason with them; there are trans people who have been allowed to transition that are living happy, productive lives as themselves. If this helps at all, you can try asking them to buy some new clothes for you.
Also, you don’t need permission from your parents to ask your friends at school to start calling you by a new name and/or pronouns. Especially if your new name could be a nickname from your birth name. And that helps ease dysphoria a lot (in my experience).
If you’re looking to medically transition, I don’t know as much. You should be able to talk to your doctor in private during your annual checkups, but that likely won’t be happening for a while because of the pandemic (aaa). Once this is over, however, that’s an option. I’m not sure how much a doctor can do without your parents’ permission, but I do know that as you get older you have more of a say in your medical care. Worst comes to worst, you can wait until you’re 18 an have complete control over your medical care to transition. While it seems like a long time, it always an option.
Everyone else here PLEASE feel free to chime in I do not have much experience this front.