If you go to Admin>Config>Destinations schools - choose the school you wish to make inactive and click on the Edit button. This will take you to another page with information about the school. On the last tab change to inactive and then click on Update to save the changes
All inactive schools will show under your current destination schools. You can make any of these active again by clicking Edit, then toggling inactive/active button and saving the changes.
If you wish any permanently removed, you will need to request this via an email identifying the school(s) you wish deleted.