In almost every case, if you are unable to add a student to your school, it is caused by the student's email already being associated with another account. This most often happens if a student changes schools and their email remains the same.
We recommend that you first reach out to the student's previous school's CritterCoin admin and request them to delete the student from their CritterCoin service.
If you are unable to do that, you can contact support@crittercoin.com with your issue, and be sure to include the email of the student that is failing.