If your school invited you to join their account, but you haven't received the invite email, you can register and connect with your school by following the below steps -
1. Visit https://www.planbook.com/#home
2. Click the "Login or Register" button in the upper-right corner.
3. Click the "Click here to sign up" link at the bottom.
4. Enter the email you were invited under and a password of your choice, and then confirm the password.
5. Click "Sign Up".
This will connect you to your school's account without requiring that email.