Just so nobody gets confused about what I'm saying (hasn't happened yet on this topic), I believe that if a kid can hit the mileage he thinks he needs to hit by running singles, then that's what he or she should do. One of the difficult things about running longer races is the fact that you're on your feet for so long. I know darned well that when you're preparing for a marathon that running a 10 miler in the morning and a 10 miler later in the day is not even close to being a proxy for running a 20 miler, so yes I am a singles lover when I can get it.
However, if the kid feels he or she can't take the pounding of the relatively longer runs each time out that is required to hit the weekly mileage on singles, then break it up into some doubles. I'm merely looking at what alternatives a kid has if he or she can't run that 8 mile run (and I'm talking about kids who are physically capable of completing such runs since no reputable coach would ask a kid to do a run that long if they knew the kid was physically incapable of it). I'm thinking in terms of what recommendations a coach would make to kids who are running on their own over the summer and may find a stretch where they don't have time to do long runs:
You know how that goes where a kid stays up too late or oversleeps and doesn't have time to do that 8 miler before having to go to work and could only squeeze in 4 miles, and then the kid has plans to hang out with friends after work, and next thing you know it's going to be dark in an hour, and the kid only has 4 miles in for the day...
We all might like to think that our runners are totally committed and will follow everything we tell them to the letter, but at the end of the day, you're probably not going to keep them from being kids on summer vacation unless you're one of the really lucky coaches. Sometimes, you have to present alternatives that allow the kid to feel like he's doing some training without making him feel like he or she's a slave to your program.
Psycho Dad does bring up a good point though, and it's one I agree with. It's not so much the number of miles but how you go about reaching that number. You certainly don't want kids to be running miles just so they can be proud of the number they see at the end of the week.
Another great point that Psycho Dad mentions is about learning how to race. My brother once told me that was the main difference between HS running and college running. In HS, they get you ready to run. In college, they get you ready to race. I really think you have to treat the sport more like a college science course than a sport and set aside some time each week of practice for a lecture session where you can teach some of the finer points of running like basic nutrition, importance of hydration, getting proper rest, proper shoe care, situational things like conditions where it's good throw in a surge to break your competition (such as on a blind turn, at the base of a hill, after cresting a hill, etc), and the like. It may not be enough to Xerox a bunch of handouts on a topic and pass them out for the kids to read on their own because most either won't read the material or won't understand what they read. Spending a little time on some of these topics can go a long way. The kids' daily workouts would be like field labs for them since you could have them observe and record data in log form.