In comparing most American kids to those of third world countries, we are consistently overlooking the couch potato factor. The few kids that have grown up playing soccer, swimming, and just plain running around are more likely to be able to handle significant training loads. The vast majority of our youth however do not have that background. They come to the start of their initial year of track/cc having done little physical activity. Shin splints, muscle pulls & strains, stress fractures, etc. occur even with minimal training loads. We like to think of 1 year as a significant amount of time, but for a kid whose body hasn't been used the amount of very basic training (playing) necessary before sport specific training can begin in earnest is consistently underestimated.
We like to think that every kid can get up off the couch and run 25-30 mpw because that's such light training, but it's not for a body that's never been used. Our training for high school level kids is going to have to be a lot more individualized. That leads to all kinds of political problems in a public school environment - when a couch potato is put on plan "B" and his previously active friend is put on plan "A" parents are going to scream and the kids are going to complain that one kid is being favored over the other. Facts only get in the way in those discussions.
In one of Renaldo's early posts on Letsrun, he talked about how young Kenyans often have 12-15 years of running around in by the time they start any sports specific training. That's a base that prevents injury and provides an aerobic base upon which a significant training load can be built.
By the time most kids reach high school in this country they are already out of shape. If we ignore this history and just have them run high mileage, the injury cycle is all but certain. I think we often forget that a skinny kid may be just as out of shape as a fat kid. Their muscles have atrophied to the point that even modest activity leads to injury.
For that small group that has grown up with a great deal of activity, higher mileage is a possibility. For the vast majority, they need to be brought along at a much slower pace than we like to believe. It may take several years to overcome the idleness that marked their previous decade. That may mean putting off high intensity training for quite some time...which requires the ability to delay gratification. In lieu of this ability, the kids go elsewhere to feed their ego's before they ever have the chance to reach their potential.
Those of you who are talking from high school experiences more than 10 years ago, I challenge you to go work with a youth track club or high school team and see if you are not amazed at the degree to which our kids have become injury prone at only modest training loads.