Hi there,
I'm also in year 11 and I do understand how this must feel. What I can say is that you're so close to the end of high school. I don't know if you're going to the same sixth form as those girls, but I would say to wait for sixth form. New people come, old people go and you can start again. You can make new friends, be a new person and forget those girls. I'm so honestly sorry that they're acting that way towards you, it's awful, and basically bullying. Do not let them see that it gets to you. Focus on yourself and your studies and I promise you, you will find a good friend, after GCSEs, in whatever education you decide to pursue, reach out and make new friends, and feel that liberating feeling of leaving behind a toxic group. I am telling you that you will be happy again.
And about prom, honestly, I'm not going either. I lost a lot of my friends and I'm so insecure in myself that I'm not going. I just want you to know that a lot of people don't go to prom, I don't know if this will make you feel better but it might help you realise you're not alone. Prom is one aspect of your whole life. I doubt if in 10 years time someone asks whether you went to prom, you'll have had so much life happen to you by then that prom will be irrelevant. If it's something you really want to go to, you can still go, look utterly beautiful, maybe just turn up and see how things go and leave if you don't feel comfortable. Or alternatively, if you really think going is going to make you feel worse and not better, just enjoy your day / night. While they go to prom, do something you love, maybe pull an all-nighter and watch a movie with snacks, or do some shopping that day just because, just try to enjoy that night as it may make you feel happier knowing that you did something even if it wasn't going to prom.
Also, there will be parties when you're older and prom is just prom in the end. Nothing that special or fancy like they make it out to be in the movies. The stories people will post? Just a small glimpse of what it is. In reality, or what it is at my school / for me, it's just turn up in dresses and eat some food, chat in groups, if you're not popular it's awkward, and I'm not popular so that's a bit why I'm not going.
I don't know if any of this will make you feel better, or feel less alone, or give you hope, but please don't let some silly girls get in the way of smashing the rest of your GCSEs and having a wonderful summer. In a few years, you won't even remember them and will have found new friends. That's part of growing up. Going into the world, meeting new people and realising what true friends are, meeting people with similar thoughts and interests as you.
You will be happy, you've just got to wait out this small storm <3