Say; 'Hey, watch what you're saying! I don't like this kind of talk.'
Look furious while you say it.
If it is repeated, frown and walk away.
Don't come out till you have completed you education and are old enough to move out and support yourself.
You don't have to tell any bunch of willfully ignorant folk a thing. Live your own life and leave them to get on with theirs.
You have been given the wonderful gift of being gay. You'll meet some marvelous people and, if you're honest with yourself, you'll have a happy and fulfilled life.
It is gay people who have kept science, the arts, and so on going. This is OUR civilisation.
Without a Lesbian, Florence Nightingale, or Alan Turing, a gay man, most of us, including your family members and school "friends" probably would not exist.
Move to somewhere where you don't have to keep hiding you true self - you don't have to screech at everyone that you're gay, but you won't have to lie about it either. Just keep quiet, frown slightly, change the subject. Let them live with their problem.