There is so much redundancy in elementary school education that you can take out a large chunk of it and still teach the same amount of information. School could be way, way denser than it is. Hell, even in high school, a full year would cover the same amount of material as a single semester of one of my college classes, and my college classes only met 3 times a week (for the same exact class length as my high school classes; or fewer than 3 times a week for longer class length).