Permit me to draw attention to the journal, Involve—A Journal of Mathematics (which I also mentioned in a related MO posting). Snippets from About the Journal:
"Involve is dedicated to showcasing and encouraging high quality mathematical research involving students (at all levels). ... Submissions in all mathematical areas are encouraged. However, each manuscript should include a minimum of 1/3 student authorship. ... Involve is a publication in between the extremes of purely undergraduate research journals, which in general are aimed at undergraduate audiences, and mainstream research journals."
The current issue is its 10th.(Disclosure: I am on the editorial board.)
