You can update your school by going to your My Account page and clicking "Edit" next to the existing school in your Schools section. Then you can either transfer any existing classes to your new school, or you can archive them.
If you teacher in multiple schools, click the "Add another school" button in your Schools section to add it. Moving forward, any classes can be created in, or moved to, this new school.
When you remove a school from your profile, you will be prompted to archive that school's classes, or transfer those classes to a school you still remain in.