This seems primarily like a marketing ploy. I signed up with my gmail account a few months ago and got my invite very soon (within 48 hours if not 24). Unless things have gotten worse, it seems more like an excuse to try to get students to sign up than Google giving students that much benefit.
Edit: Apparently the situation has gotten significantly worse since I signed up, which is not that long ago.
I think half of the reason Google uses invite-only beta services so much is to make people want the service more. People desire things more when they are scarce: http://en.wikipedia.org/wiki/Scarcity_value
I'm sure that adds something, but half seems a little high. The invitation tree must be highly valuable information when it comes to fighting spammers.
Things have gotten worse, though, it seems; when I signed up it took about a month before I got the invitation, and I know others who've been waiting ~two months now and still haven't gotten it.
I'm not saying that Google Voice isn't useful to students. I'm saying that students could have pretty much gotten an invite within a day even before this program just because the invite lag is pretty short. But it's easier to get people to blog, post to hacker news, etc. about a "special" program, so they more-or-less invented one.
Edit: Apparently the situation has gotten significantly worse since I signed up, which is not that long ago.