The most common reason for this is that the teacher already has a CritterCoin account registered with the email that you are using to invite them.
In many cases the teacher may have created a School account rather than waiting for you initiation. If this is the case, ask them to...
1) Sign into their CritterCoin account
2) Switch to the Admin view is not already in it.
3) Click "Settings"
4) Scroll to the bottom and choose "Delete Account".
After they delete their account try inviting them again. If the problem persists please contact our support at firstname.lastname@example.org