In July of last year, a Mississippi high school sent 11 marching band members to the hospital after they collapsed during practice.

I was in the marching band in high school in the Mojave Desert, where the temperatures would regularly break 100F/40C. You lose a lot of water when you're playing an instrument (they don't have spit valves for nothing), and marching bands still maintain a lot of traditions from their military origins, so you do a lot of push-ups. The thick wool uniforms you wear during performances don't help anything either -- we had elaborate rituals involving baby powder to try to stay dry inside the uniform, which was harder than you might expect considering how dry the desert air is. We'd have a few kids pass out during band camp each summer, but I can only remember once someone went down during a performance -- she marched right off the field and collapsed on the sideline, and was fine after some fluids and rest.