Schools that use different e-mail address domains (the part after the @) for students and teachers may result in an error when some students try to add coaches.
Many school-supplied e-mail domains are restricted domains which limit the coaches that students can add in order to protect the privacy of students' data.
If you are trying to coach a student account registered to a different email domain than you, the student's parent needs to adjust their settings in order to enable this. Parents can take the following steps on their children's accounts if they want them to be able to add coaches outside of their domain:
1. Go to https://www.khanacademy.org/parent to view the children attached to your account.
2. Click on "View Summary" for the child in question.
3. Click on "View Settings" from the next page.
4. Scroll down to "Coaches" and enable your child to add coaches to their account.
5. Once this is enabled, your child will be able to approve coach requests themselves, via their notifications. They will then be able to be coached by accounts outside of their exact email domain.