You simply can't. Colleges change very, very, slowly, and the processes, committees, diverse interests involved in every decision make it very hard for any small group of people to accomplish change, even in the faculty or administration.
And while Ubuntu is free, change is costly. How many hours do you think it will take to install Ubuntu on all of the machines? How much time to train IT staff that could do everything they needed to do on Windows but are unfamiliar with linux? How much time with the administration spend in dealing with complaints from students, staff, faculty, and parents who don't understand the motivation for this change?
Oh, and what if some part of a mission-critical management Windows application has a bug that only shows up when run in Wine. How much money would be lost then? That probably won't happen, but how sure would you need to be before authorizing the change?
The problem with the question is that you are thinking as an individual, and change is cheap for individuals. For large orgainizations ... well, there is a reason that my employer still uses Windows XP here in November of 2011.
Addendum: Please keep in mind that as far as we know, here in 2011, there are no colleges on earth that have made the switch to Ubuntu. It isn't necessary that we conclude that it isn't feasible (though I have, for other reasons) but the other answers here either haven't been tried, or have been tried and failed.