"What would help, in my mind, is for kids to be able to learn each subject at their own, separate pace--one "micro-skill" at a time, advancing to the next only when all the micro-skills relied upon as a base for the next micro-skill have been mastered (100%ed). [Think of it sort of like an unlockable "tech tree" of education.]"

This is exactly what the Khan Academy is doing. I believe there's at least one school (not sure if public or private) where they're experimenting with that model. See:

http://www.ted.com/talks/salman_khan_let_s_use_video_to_rein...

IIRC, the students in the class take most of the lessons via computer/tablet and move forward once they've answered a certain number of quiz problems correctly. The teacher can monitor the progress of each student, and if one is having difficulty than he/she can get some individual help from the teacher or another student.