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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.
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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.
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