To add Grade Level to a list you will first:
  1. Navigate to Lists > Manage Lists
  2. Click the Manage basic and advanced lists block under List Actions
  3. Locate and edit your list
  4. Add User Register Under Constituent Information
  5. Scroll down to the Object Category Platform and Open it
  6. Select Grade Level 
You can then use this to display the grade levels under Display Fields or Filter by it under the Filters object.