Year 11 isn't easy - to be honest I think having someone by your side is important and being alone is not fun and it pretty much sucks. I managed to make a friend in Year 9 and I still hangout with her and talk to her now in college (I'm in my first year and she goes to a different college)
Have you tried talking to anyone in your class from higher bands? There might already be friendship groups formed between them but try talking to some people, maybe starting conversation by asking for resources or pretend you don't know the answer and ask for help. Always make sure to smile, because that's literally helped me so much to seem more open to people.(When I was in year 11, my friends weren't in any of my classes apart from 2, so I had to be by myself all of the time, however I always made sure every interaction with my classmates was positive, so I became known for being a generally nice person. I always made sure to smile at everyone.) Does your school have any clubs? If you have no one to be with at lunch perhaps there's lunch time clubs you can go to instead, or after school as well. You might meet new people there.
It's alright to keep to yourself because after year 11 finishes trust me no one even remembers most of the details and it doesn't particularly matter. You should make the most of all the time you have alone and use some of it to revise, maybe hangout in the library, read something if that's what you're into. If you want to go to college or sixth form you will meet so many new people and should definitely take that chance when you get it. For now just hold on tight and finish your GCSE's and make the most of them. Leave those fake friends, they seem like they don't particularly care for you and you're wasting your time with them. You can have a better time by yourself than feeling uncomfortable with them trust me.
As for the holidays, I get that it can literally drag when you're stuck inside. Is there any relatives like cousins or relative's friends you can hangout with? Maybe you can hangout with you parents (if it's not awkward for you) and do something with them, or siblings if you have any. Maybe your parents know someone else's daughter/son's parents and can link you up with someone to hang out with.
Additionally, you can always go out for a walk and listen to music. You could go for a walk every day as part of your routine so the holidays don't seem so bad. Getting some fresh air and taking in your surroundings is much better than being stuck in your room (hopefully your parents let you go out for things like that). You could have your own picnic somewhere, in a park perhaps. If you're into creative stuff why not practise drawing your surroundings. For example if you're in the park, try drawing what you see (there's lots of youtube tutorials on drawing and stuff like that)
Hope your situation is Ok and hang on tight because once you're out of high school it is a lot more freeing and not as restrictive as being in the classroom
