I googled the "Bloom two sigma phenomenom" and... correct me if I am wrong, but I parsed it as:

"If we keep teaching students each lesson until they understand, and only then move to the next lesson (as opposed to, I guess, moving ahead at predetermined time intervals), they will be at top 2 percent of all students".

What exactly is the lesson here? The weaker form seems to be -- if students don't understand their lessons, it really makes a difference at tests. (I guess this is not a big surprise.) The stronger form seems to be -- in standard education, more than 90% of students don't understand the lessons. Which suggests that of the money given to education, the huge majority is wasted. Okay, not wasted completely; "worse than those who really understand" does not necessarily mean "understands nothing". But still... I wonder how much additional money would be needed to give decent education to everyone, and how much would the society benefit from that.

Based on my experience as a former teacher, the biggest problem is that many students just don't cooperate and do everything they can to disrupt the lesson. (In homeschool and private tutoring, you don't have these classmates!) And in many schools teachers are completely helpless about that, because the rules don't allow them to make anything that could really help. Any attempt to make education more efficient would have to deal with the disruptive students; perhaps to remove them from the main stream. And the remaining ones should learn until they understand. Perhaps with some option for the smarter ones to move ahead faster.