To add/edit enrollment rows:
  1. Navigate to Core.
  2. Using People Finder, Search for the user > Select the user.
  3. Select the Access Tab.
  4. Select the Pencil Icon beside the Student Persona. 
  5. Select Edit Enrollment.
  6. Select Add Another Enrollment..
  7. Select Save

To bulk add future enrollment rows, please refer to How do I promote, repeat or withdraw students at the end of the school year? .