If you have accidentally sent a join request to the wrong school you can cancel the pending request from your teacher account.
To cancel a pending join request, you will need to:
1) Click on the Cancel request in the message that appears on your dashboard.
2) You will then be directed to your account profile page. On your profile click on Cancel this request.
3) You can now either click Done to go back to the dashboard and be automatically directed to the Find your school page or click on Find my school on your profile to start the process.