To join a school, click the Search option in the orange bar at the top right corner of any page. Type the name of the school you're looking for, plus the state and/or city. Once you find the right school, Click on join to the right of the school.
If you don't initially find your school, try possible alternatives—neighboring towns, consolidated school districts, similar school names, etc.
You can also join another school from My Profile, which can be found in the drop down menu next to your name/picture in the orange bar at the top of any page. Simply click the Schools tab, then look for the Add another school section on the left side of the page.
Using Mobile? Click the spy glass icon next to your picture in the orange bar at the top of any page and type in the school you're looking for. Once you've searched for the school, click the name of the school you'd like to join. Once the school page loads, click the + icon to join the school.