Because of this one girl I used to be friends with, my reputation got slandered and ruined. Now I feel like everyone at school hates me. Half my class has even said that they hate me. How do I regain my reputation?

Hey, I’m so glad you reached out to us! I hope you’re doing okay.
Feeling like everyone hates you at school obviously sucks — you have to spend so much time there, and when you aren’t comfortable it can really start to weigh on you. I do want to say though that since I don’t know you, or any of the kids in your class or anything, I can’t really give you concrete advice on how to win them back!
Still, sometimes situations like this aren’t as bad as they seem. I can say that this has happened to me a few times and from my own personal experience, I found that if I usually wait it out people tend to come around. Have you tried telling someone else about it though? Maybe if you had the chance to have a conversation about how you’ve been feeling with everyone in class with another friend, they could help you. Even telling your family or a teacher might help you feel like you have someone on your side. Other that, I would just ask you to take care of yourself! Be sure to get enough sleep and food, and spend some time doing things you like. When I get too stressed, I just try to distract myself and sleep on it, and half the time it makes me feel better.
Try to find that support in peers or other people who don’t believe the rumours and if you have a hard time doing that, just remember you can always find that support in yourself!