My HS XC junior year I went from middle of the pack runner to almost winning state by running 45-60mpw with a 13 mile long run (22% to 29% of my weekly total). Previous year I had run 45 mpw with no run above 7.
My senior year our coach capped our long run at 10 miles (16% to 22% of my 45-60 mile weekly total). All other training stayed the same except one additional easy morning run per week. I ran 20-25 seconds slower on the same 5k XC courses all season and barely top 10 at state.
I'm not saying HS runners should do 13 miles for a long run, or a long run at all, just that it worked for me.