Groups for Schools | Facebook Help Center

How do I find groups for my school that I might want to join?

When you visit your school\’s main group, at the top of the page you\’ll see a pulldown menu that lets you see: suggested groups, your groups, all groups and friends\’ groups. If you see a group you’re interested in, click the Join button that appears next to each group.

There is also a search field where you can find a specific group.

via Groups for Schools | Facebook Help Center.