Teens may be quick to adopt new means of digital communication and social networking tools but they're definitely not clamoring to join the Twitter ranks.
As discussed in my PodCamp London session (PDF version of slides here) and expanded by Dave Fleet in his post "Youth vs. Adults: Strong Ties/Weak Networks", teens seek stronger ties that just aren't available through Twitter.
Twitter, as compared to Facebook or MySpace, is much less personal and is more frequently used to build large, weak-tie networks. In fact, it's estimated that over half of Twitter users are over 35.
The above facts and observations lead me to frequently recommend Twitter as a tool to strengthen ties with alumni as opposed to communicating with recruits or even undergraduate students. As always, having sound social media policies in place and using a segment-specific mix of tools and platforms is always the best approach.