Is training for and ultimately running a half marathon good training for a high school cross country season?
I recently trained for and ran in my first 1/2 marathon, and I can honestly say that it would be an EXCELLENT idea.
You are basically building endurance for a 1/2 marathon, but not doing much speed work. This is fine for preseason high school cross country training.
If you are worried about losing a little speed because you will be doing longer distance runs in preparation for the 1/2 marathon, add a Fartlek Workout or some intervals once a week into your training.
Here's a video about some running workouts that might be good for your training. Good luck with the half and the CC season.