Okay guys...I'm 30 years old now. I ran a 8:46 3km in hs (say that's worth about 9:23 or for 3200m...I don't know exactly but close). A little slower than our 9:15 example. Keep in mind though I was also already running 32:05 for 10km and 1:13 in the half marathon in hs too.
When I was 26 and focused on marathons at Hansons I ran 2:16:52. Now I'm an old and slow ultra guy, and have only run a handful of sub 2:20s in the last year (but I did get 16th at Boston last year). If I focused on roads and more half/10km speed I always though maybe I could run around 2:15...but I haven't and likely never will....okay, enough excuses!
I'd say anyone who runs 9:15 in hs has quite a bit of talent. That is a very good time that will get you noticed by many DI schools. But can you back that up with solid cross country performances? Are you more of a 3200/5km runner in hs or more of a 800/1600/3200 runner? That might show some natural talent for "strength and longer distances" rather than natural speed/high Vo2max on just the track.
To do well in the marathon it generally takes many people years and years of 4000+ miles/year. You need to have very dialed running economy and a well trained lactate threshold. When I ran in the Olympic Trials in 2007 and 2012 I didn't personally know a single guy at the race who ran less than 100-miles a week. (I'm sure there were some, but you have to be pretty freaking talented to pull that off). The majority of guys running sub 2:19 or so ran pretty darn well in college and continued running for years after college at a pretty high level.
So the key is really consistent training over time (fairly high mileage), not getting hurt, and probably running a lot of 100+ mile weeks. If a 9:15 kid can do that for 2-6 years after running all 4 years in college, then I'd say they have a very good chance of at least going sub 2:20 (and some could go sub 2:15 for sure).