First up is a performance of Star of Valor, uploaded by the AFA Symphony Orchestra of the American Festival of the Arts Summer Music Conservatory at Pershing High School in Houston, Texas. They do a really nice job balancing the overlapping melodies and negotiating the tempo changes. Their intonation is really good. And just look at the concentration in their faces.
Here's a really nice performance of Westward Motion, performed by the CMS 8th Grade Orchestra. Again, there's no indication of where they're from, geographically. But they do a nice job with the piece. My only quibble: they clearly have timpani, chimes and a glockenspeil on stage and they're completely unmanned and silent throughout the piece! The percussion parts are optional, but I always prefer to hear them whenever possible.
I've saved a really good one for last: check out this performance of Gauntlet from the String Quartet Program of Northern Colorado Works in Progress concert. It looks like an all-ages group and they do a remarkable job bringing the emotion out of the piece. This is definitely one of the best performances I've heard of Gauntlet. The whole concert is great, but Guantlet starts at about 12:10.
There is another video of this same performance, just of Gauntlet, taken from a closer angle at stage level, but I wasn't able to link it to this post. It should be easy enough to find with a quick YouTube search, though.