They can do this by clicking Go To -> Teachers and clicking the "Invite Teachers" button at the top of the page. 

You can also manually join your school if you have your School ID number. 

To enter this number, click your display name in the upper-right corner and select "Account". Here, enter the number in the "School ID" field and save the changes. This should connect you to the school after signing out and signing back in next. 

If you are still having trouble, please reach out to us at support@planbook.com.