tl;dr if you provide each student with the appropriate kind and quality of instruction and the amount of time available for learning then the majority of students will achieve mastery.
I had two of my kids in a mastery school. The school didn't "do" mastery very, well, but it's a lot better than public school. Students learn different parts of a subject at different rates. By tuning the time the kids had to learn a sub-subject, they could fully master the subject, and at an accelerated pace compare to public school.
If I could get Khan-academy + peer socialization + baby sitting "in one room", I think that'd be ideal.
