Classmates.com specializes in connecting old schoolmates.

You create an account, then choose your school from the site's list. You enter the year you graduated, your age, and a bit of other identifying information, and the site matches you with other graduates of the same school, particularly those in the same graduating year.

From within Classmates.com, you can contact other classmates, and email them directly.

(I'm still gathering information on all you can do within Classmates; I'll update this lesson once I learn more.)