MDMA should be used as a supplement to an enjoying an experience. An experience should be fun because you had a good time, and not just because you were rolling(if that makes sense).
At 15 years old, you need to learn to have fun sober. When you're that young, your mind is still developing and being in high school is one of the best times you'll ever have. You don't need a drug to enhance that for you.
After you've become an adult(above 18) you may decide for yourself that you want to take your experiences a bit further with the help of MDMA or similar drugs. I know for me personally, 15 was still a somewhat awkward time, but it was one of the biggest learning experiences of my life. I learned to talk to girls, socialize with random new people, and become more comfortable with myself. By introducing drugs into the equation, you're not helping yourself with any of that. Sure everyone is more friendly when they're on MDMA, but what happens when the drug is gone, when you no longer do it? Then you're stuck learning how to socialize sober all over again.
My advice is pretty simple. These are some of the best days of your life. Enjoy them sober, and use this time to learn more about yourself and learn to become a better person. When you're a few years older, reevaluate the situation, but don't do it just because your friends do it.