First check in Core> User/Access> Profile to see if the Student has a Parent Listed in the Relationship Settings (this does not go by List as Parent and Parental Access, just the Parent Relationship. There is a current Idea in the Ideas Portal  to make it List as Parent and Parental Access as you may have some Grandparents that are the Students Legal Guardians.) Alos check the Students Roles and see if they have the Content Editor Role. If  Students are also Content Editors the System does not flag them as "Students" within School Forms.

To allow the parents to submit you can temporarily remove the Content Editor role from the Students that have No Recipients so the Parents can fill out and submit the form correctly. After the School Form is Submitted, you can add the Content Editor Role back to the Student.

To remove the Content Editor Role you will need to know what kind of Group they are a Editor of such as a Community Group, Activity Group etc.
  1. onCampus
  2. People
  3. Group Type (ex. Activity, Community)
  4. Group Type Content Editor Access (ex. Activity Content Editor Access)
  5. Select the "Display By User" Radio Button
  6. Click Edit next to the Student Name
  7. De Select the Group (make a note of the group so you can add it back to the User after form submission using the same steps above)
  8. Click Save