If you are a teacher and your school's invitation isn't working, the most common reason for this is that your email is already in use in CritterCoin in another account.
If this is the case, you can delete your unwanted count and ask them to re-invite you.
To delete your Account
1) Sign into your 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".