I've visited schools several times (one of my friends is studying to become a teacher), and the problem isn't the length of the day, or how many days there are. The problem is kids who don't listen or even care, bad teachers who don't care about the kids, lazy and disengaging lesson plans, and a generally incompetent system of higher-ups who don't know anything about teaching kids. 20 extra days wouldn't make a big difference if most kids can't even pass as proficient in reading and math. Seriously, I can't think of many things easier than being proficient in a school subject.
Kids in East Asia and various other regions of the world perform better than American kids because their system is better and their parents push them harder to succeed.
