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.

To create a list that displays information populated in custom Publishable Fields:
  1. Go to an application persona (ex: Core).
  2. Analysis > Lists 
  3. Manage Basic and Advanced Lists 
  4. Select Lists Templates.
  5. From the Template Category drop down, select Constituent Information.
  6. Navigate to page 2 of the list templates.
  7. Click View/Copy for List: Users With School Defined Fields.
  8. Enter a Name for the list.
  9. Click Save.
  10. Click Preview in the bottom-right to view the list results.
Note: 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.
To delete a custom field:
  1. Go to Core > Settings > Custom Fields.
  2. Under Publishable Fields, delete the text in the Field Name. This deletes the field.