Permit me to draw attention to a relatively new journal, Involve—A Journal of Mathematics. 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."
I would guess that nearly all of the papers that have appeared in its first nine issues reflect work supervised by a faculty mentor, and a good percentage are coauthored with that faculty member (and not infrequently with more than one student).