First of all let me say again what I said in my first post. I am NOT against doubles, I just believe that if you can handle singles they will be better for you. I am basing this off experience. But I do understand what works for me does not work for everybody. Normally when I increase my overall mileage I start by adding doubles then cut them out as I get used to the mileage. I feel stronger when I am running singles. Also I just enjoy running longer runs. About running a shakeout in the morning... I am still in school and I have 8am classes everyday. When I have to stay up late studying and such I don't really want to wake up earlier and cut out sleep. Sleep is a very important part of recovery. I believe sleeping more will be more beneficial to me than a 20 min run. So I sleep.
To bring the topic back to should a hs runner be doing 20 mile runs... I don't really think they need to do them to be a succesfull hs runner. Though if the person knows they want to be a 10k runner in college, which I'm assuming means he knows he is better at the longer races, then why not go ahead and do the 20 mile runs. If you enjoy them building a big base is important and it will help you in the long term. If the hs runner wants to move up in distance when he graduates college then he is more than used to the longer runs and it will be an easier ajustment. But don't do them before doing your 14 then 16 then 18 milers. Running is all about progression. As you progess it becomes more and more enjoyable. That's what makes running so much fun. So go for the long run, but remember progression and build up to it.