That's obviously going to vary by quite a bit due to several factors... how knowledgeable the coach and athletes are, how hard coaches push the kids, how hard kids push themselves, how much injury prevention work is incorporated into the program, how well the kids eat/sleep/recover, how frail/injury prone the kids are to begin with, what kind of terrain the kids have to train on, etc.
FWIW, the injury rate on my team when I was in high school and we ran a healthy amount of trails rather than just concrete was low - maybe 3% at most. The HS got a new coach after I graduated, and almost all the runs were on concrete or hard trails rather than soft trails, and pushed the kids harder, and the injury rate shot up to at least 15%. (%'s are rough based on memory, I'm not specifically counting out who all were injured)