If you go to your profile page you can delete your Classlist account. At the bottom of the page you can choose to leave a school.
If you are a member of more than one school you will not be deleted from the second school. Therefore if you are joining a new school it is a good idea to join it before leaving the first! You can do this by clicking on the school name at the top of Classlist, and applying to join a new school. Your profile will be transferred but you will need to add your pupils to the new school.
Your profile is at the top right on a computer. On the phone open the menu to find your profile. The menu is found at the bottom right (3 horizontal bars).