The best high school student incentive ideas involve food and drink.
Another incentive, which my students love, is the opportunity to earn eating/drinking privileges. My students are not allowed to eat or drink in my class, so this is a real treat. They will do whatever it takes to earn this privilege. I use it as a reward for grades, but you could easily use in as a reward for improved behavior.
If you extend an offer to students to better their grades, they see that you care and are trying to help. Most understand that you will only help if students do their part. Once a student sees that you care, getting them to do what you want becomes much easier.