My first thought is Spoon River Anthology but it's not super interesting, IMO.
I recall French has an anthology called the Bully Plays that has a monologue play in it.
I just did The Swing of the Sea by Molly Hagan this past year. While not entirely monologues, there are many long ones in it, and it is well written and perfect in the middle school/high school context.
Jane Martin's Vital Signs has a few pieces for males, but most of the monologues are for women.
These are vignette plays that have scenes and monologues:
Peter Dee's Voices from the High School/Voices 2000Brad Slaight's Class Action and Second Class