In her Back to School Stress-Management Toolkit for Teens, Melanie Greenberg suggests that one way to deal with school (or any other stress) is to distract yourself. The first part of this is to write a list of things that you can do to distract yourself. The second part is to do one of them. This engages the "on task" part of your brain instead of the "worry" part. The trick is to choose short activities that won't take too long. Avoid computer games, social media and netfix - they are sure to consume too much of your time.
In an article on how to deal with pessimistic thoughts, Dr Mike Dow said that the single most effective tool is gratitude. And not just listing things that you are thankful for but writing why you are thankful for them as well. You can read the full article here.
School hasn't gone back yet but these make awesome holiday snacks too.