This is pretty amazing; it's the Poteet High School (Texas) Band performing Emerson Lake and Palmer's Karn Evil 9. On many marimbas. And doing it really well.
(I first saw this two years ago on the Poteet Pirate Band web site, but unforunately it wasn't up for very long. Luckily somebody grabbed it and made it more generally available.)
Of course adapting progressive rock to alternative performance realms is potentially trecherous. Here is Yes' Roundabout as interpreted by the Hastings High School (Minnesota) Show Choir: