We remind the students not to litter, but they don't listen to us. What do we do?

Teach them about the harmful effects of trash when it gets into water, and when wildlife get stuck in it or try to eat it. Search for microplastics, Midway Island, and the Pacific garbage patch if you need to learn about these subjects. You could also have the kids do reports on subjects like these. It's a great, real-world science lesson. Then, ask the students to come up with ways to deal with the problem, especially at the school, where they have control. Help them choose the most practical solutions and put them into practice. Don't be surprised if the students start organizing monthly cleanups and reminding each other. It's more effective coming from each other than from you.