The 90-day window
Deleting moves the school out of everyone's view but keeps it recoverable. If you change your mind within 90 days, the Owner can restore the school along with its members, classes and assignments.
After 90 days
The school is purged. Memberships are removed, school-created student credentials are wiped, and the record is gone for good. This cannot be undone, so export anything you need first.
What is not deleted
People. Teachers, parents and students keep their own Meta Typing Club accounts and their personal typing history whether or not the school exists.