Before deleting any Publishable Field under Custom Fields, first make sure the field is not populated in any user profile.
If data appears in the custom field you want to delete, manually remove the data from the user's contact card first. You can create a list to view which users have the field populated.
If you need to remove data from 20 or more records, you may submit a data fix request to BB K-12 Support.
After you remove the data from all user contact cards, then you can delete the custom field.