It is being done.

A school in Oregon is implementing proficiency-based student-centric learning where time becomes the variable and learning becomes the constant. We've known for a long time that this is the right thing to do. It's a shame, the main thing stopping this from happening everywhere is "cultural inertia", and in some cases a sense of security for some adults taking precedent over the best interest of future generations of learners.

Here is the article:

http://www.statesmanjournal.com/print/article/20100208/NEWS/2080332/Students-in-charge-of-own-education