To join a school or college, click the Search option 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 Join next to the school name, then fill out the requested info to add your name to the class list!
If you don't initially find your school or college, try possible alternatives—neighboring towns, consolidated school districts, similar school names, etc.
You can also join another school from My Profile. Simply click the Schools tab, then look for the Add another school section.