Les Miserables at Capital Playhouse in Olympia, Washington.  It was a youth production, but saying that makes it seem like it was less than it really is.  Honestly better than almost all professional theatre that I have seen.

I know this is an old thread, but I just came upon it and thought it might be interesting to bring it up to date.

I recently saw two separate high school productions that were top-notch.
 
Aida, a show I've seen 3 or 4 times, including  the travelling Broadway production and that I often describe as "really not my favorite show in the world," was performed by students of Cambria Heights Senior High School (Patton, PA) that was simply stunning from start to finish.

Shortly after Aida, Central Cambria High School (Ebensburg, PA) presented Godspell, which simply blew me away.  Being an athiest once a Catholic schoolboy, I don't particularly care for the content of the show but know the stories more than well enough to follow along, and I have to say, this was the most magical show I've ever seen.  I dare call it a letter-perfect performance.  The actors were polished, the singing solid, the choreography incredible -- one particular scene paired two male students, one a lifetime student of ballet and the other a complete novice  -- such that each's ability complemented the performance of the other instead of overshadowing it or dragging it down.  It was kind of a back-and-forth between classical ballet and a mix of Charlie Chaplin or Buster Keaton that just played off each other perfectly.  Simply wonderful! The "scene changes" were non-existent -- the show just "flowed."  It's a pity I opted to go the last night of the run -- I would have *loved* to see them run it again.