My Friends Are Big Users And Take Advantage Of The Fact That I Can't Say No!

I'm a girl with a lot of friends and a handful of real friends, not even that many! I am one of the best in the class and I get some of the highest grades in my class (my grades are mostly 5 and 6 and from time to time 4). During all these 7 years, my classmates use me to prompt them for tests. Because I can't say no, because I'm still afraid that they will come to the conclusion that I'm rude, I keep telling them. I feel like they are my friends just to use me. When they are sad, they complain to me and film for 100 hours, and when I am sad, they tell me to shut up and stop filming. I always reassure them that they are sad, because as someone is sad or crying, even if we write to each other, I feel sick somehow. Before I realized that these "friends" of mine my mother used me, she always told me not to trust them and I was still like "my mother is filmed a lot, I won't listen to her" and today at school, when they used me again for the math test, I thought about my mother's words. I didn't help them and pretended I didn't know and my classmates kept yelling at me "come on we're friends and you don't want to fail, we know you know" and I told them I forgot that, but when I wrote it on the test I think they saw it and copied it, at least the people who were my group. In principle, when the test starts, they exchange their tests to be one group and our teachers are blind and do not notice them, fortunately not all teachers are like that in all subjects. Outside of school, I also have similar traitors! They use me to help them with other things. Even of my 3 best friends, 2 of them are not real and they just lie to me and think I don't catch them. The other is a nice girl with the same problem as me, but I can't share how I feel, because with these 3 girlfriends we always go out together and we are separated once. If I tell this girl, she may not want to tell them anything. I don't want to share with my mother because she is busy enough and I don't have anyone. Not only do I have problems with my brother and other relatives who make me very depressed, but that's it. I feel sad and happiness no longer reigns in my soul because of the problems I have with my traitors and family problems. I'm afraid that in the 8th grade the new school will have the same users. I'm even afraid of the fact that some people want to enroll in my class (they don't know where I want to apply because I keep telling them I don't know. At least it's not because of me).