This can be corrected by undoing the Incoming Student enrollment and re-enrolling them in the correct grade:
Note: The below steps will work for users with the Incoming Student role only. 
  1. Go to Core > People Finder.
  2. Search for and select the student.
  3. Select the Access tab.
  4. Click the Pencil icon next to Student.
  5. Select Undo Incoming Student enrollment.
  6. Click Confirm.
  7. Go to onBoard > People Finder.
  8. Search for and select the student (now a candidate).
  9. Verify the Entering grade is correct (if necessary, change candidate entering grade).
  10. Go to Candidate > Process > Enroll/Inactivate and re-enroll the Candidate in the correct grade.