You need to go to your school nurse/guidance counselor/personal doctor/planned parenthood clinic to ask about how to be put on birth control. Or, if you aren't scared of your parents' reactions, ask your mom.
Once you start having sex, you will need to see a gynecologist. They make sure everything is ok and that you aren't harming your body.
I strongly advise you to get some condoms. You can get them just about anywhere and you don't have to be a certain age to buy them. Planned parenthood will actually give them to you for free.
Can you imagine getting pregnant at 11???
I got pregnant when I was 21, ten years later, and it's still hard! Raising children is a constant strain. It's a strain on your other personal relationships, a strain on your emotions, your physical body, and strain on your nerves.
Plus, you wouldn't even be able to get a job for 5 more years.
You still have 9 years of school left, you're not even in middle school! You must be in what, the fifth grade?
How old is your boyfriend?
What kind of parties are going on where there are 11 year olds having sex in the bathrooms??
Where are your parents?