NASA's Innovative Partnership Program (the same area that has the Centennial Challenges) and its Education Program are holding an essay contest for students associated with the NASA 50th Anniversary. This was noted at both Space for All and NASA Watch.
I was one of the people that NASA Watch mentions as having to accidentally stumble across the announcement to know about it (when I was checking out some other things at the IPP program about a week ago). Unfortunately, I didn't post about it when I should have (I consider such contests as in the scope of this blog, although the blog is mainly about hardware-related contests), and in my concentration on some other post I even forgot to add it to my "catch up real soon" list. Thanks to the 2 sites above for doing a much better job.