studentdesigners.com/
This has a basic principle of you make an account and upload an online portfolio with the idea that students and professionals will see your work. This for me personally doesn't quite do it as a 'student' aimed website and has no real differences to one like say behance which is for professionals.
graphiccompetitions.com/
There's also plenty of websites which just deal with competitions, a bit like a hub where people can post and discover new briefs but again there not particularly exciting or original. It's nothing which involves a community for young designers.
Forums show my idea of a design 'community' which isn't just about finish but personal practical and theoretical development as well. However forums have always bored the hell out of me and trawling through visually generic posts has always left me uninspired. Even with the forum presence I think it can be built upon.