Most students I know who know what git is are technically adept enough to install Trac and svn, or their own git or hg central repo, and would rather do that than pay $7-$12/month. So I doubt you'd be losing many paying customers by offering free student accounts. But once people moved on to other projects, they are more likely to go with what they know, which will by then include GitHub.
If I had a free github/alternative account I'd probably give it a shot and give that my time. Although more for curiousity sake... git doesn't integrate with the VS.net IDE which I spend a lot of my time in.
They wouldn't lose me as a paying customer because I never intended on paying in the first place (as evidenced by spending more than $7 of my time in setting up an alternative). They may gain an interested follower in the long run.