With a child of my own interested in running a marathon, I researched marathon training programs for youth in the US (there are some!), injury rates and how these compare to high school running programs.
Sorry Jesus. I'll take the snark down a dozen notches. Jack Daniels has a lot of stuff about optimal training for youth - he did a lot of high school development work but he was performance focused rather than injury focused. I'll take a look in the book case and see what I can find. Tim Noakes is always a good starting point (although he is very out of fashion these days).
Full disclosure: this is not my area of expertise. I was an organ physiologist who was serious about running for many years. So I was sort of professionally adjacent to these guys and paid attention to the literature, but I am not a subject expert.