In New York State students attend school from Labor Day until the third week of June. That is, today was the last day. State Regents exams went on this week. Summer school begins the week after 4th of July and continues until the second week of August. Therefore, some students and teachers may have 2.25 months vacation, and some only a few weeks.
The average class size is 20-25 in elementary school, 30-40 in high school. To increase class size, we would have to increase room sizes. I have been in schools where there are not even enough desks -- one student sits in a folding chair. I don't think fire code will allow for more students.
I have read some of this material by Stevenson and Stigler. I notices that their teachers (in Japan) do not seem to have lunch duty, schoolyard duty, clubs, fish fries, candy sales, study hall, hall duty, and all the other demeaning jobs assigned to teachers in this country. What other professionals have to do jobs totally unrelated to their training? What other professionals have to pay their own dues for memberships in professional organizations, pay for subscriptions to professional publications, buy their own materials and manipulatives?