Indeed. Quantity isn't the issue, especially when data available shows that the US school kids has up to, or more than, twice the number of hours spent in school. I know my kids in third grade here have longer days than I did in Sweden in the third grade by a rather significant margin. What is also rather apparent is that US school kids tend to spend less weeks in school, but with longer days. It's pretty obvious for anyone that works that mornings usually are more productive, by far, than evenings. The same is true for schooling. Adding a few hours to the day will result in even less efficient learning and bad bang for the buck. Add days but lower the number of hours per day, and quality will go up even if the hours spent at work goes down.
Heck, it's not uncommon in the nordic countries at least go have 7x5 work weeks (yes 35 hours) and as a result gain overall productivity and quality of work performed. More doesn't even equal better. In fact, most of the time it equals worse.