I think the fastest mile will be run by one the 800m guys, if you can run a sub-1:50 800m and you are a very good XC runner, I don't see any reason why you can run a mile. However, I don't see anyone going under 4, but I can see a half dozen guys going sub-4:05, which in the grand scheme of things means more than one singular sub 4. In fact I would not be surprised if one or two run guys in the 4:02-3 range. Every year we get a bunch of kids who seem capable of sub-4, but it never happens. There is a reason for that, which partially lies in the fact that great times are usually run in time trail type races where one kid who is head and shoulders above the everyone else and takes control from the guy and races against the clock (ala Ryun). The other situation is racing against pros and getting pulled along (ala Webb). When you get several H.S. kids in a race of equal ability, they generally get in each other's way or end up racing each other instead of the clock. In fact Fernandez almost got caught in that trap for half of last year's NON 2-mile, the record was not in hand until he separated from the crowd and starting racing the clock, which is what he did from the start in his 4 minute 1600. The really good news is that there are several kids in the pipeline that in the next year or two, you might see a couple of sub-4 if their development trajectory continues.
Don't sleep on the Johnson kid from Ocean City NJ, if he is healthy, fit and gets in the right race, I can see him as one of the sub 4:05 guys.