I generally don't do single-leg work too much, its worth a shot to try it out, but I always had great results with the basics, especially with young kids. Imbalances tend to work themselves out when you get a kid stronger.
plus I'm not Mike Boyle, I believe in the power of the squat
If I had this kid more than 2 days a week 1 hour each shot, I would experiment more, but results need to happen so I don't usually try out new theories in situations like this, I stick to what is proven to work.
And I always go speed/explosive/skill/lifting. My reasons are because I don't want to train a kid to be explosive when his muscles are fatigued from a 8x2 box squat session.
My reasons for box squats are simple. I don't give a shit about his back squat, athletes don't have to be powerlifters, they have to be strong and explosive, having a strong box squat always translates into a strong squat, so why not get the best of both worlds by adding the element of explosiveness to it?
I will argue that I can train high school kids just as effectively as most, if not better, because I will work more on the core movements and not try to prove how smart I am or how many apparatus I can put them on.
Pro-athletes; however, is a different story, I am working on getting more knowledge so I can take a good athlete one day and make him better... that takes time and experience and I hope one day I will have that chance.