As many others have said, reason we are lagging behind the industrialized nations in grade-school education is quality, not quantity. Two more months of mediocre education is not going to produce better students.

In addition, several have made the argument that workers don't get the time off that students do. So? When you enter the work force, you choose a field that has at least some interest to you. In school, you study whatever the state says you are going to study. If you hate your job, you can search for a new one and leave. If you hate some element of your school, you have no choice but to deal with it. As a recent graduate of an Ivy League school, I did pretty well for myself in grade school. However, I didn't enjoy every minute of it. In my junior year, pretty much the only thing that kept me going by the spring was the knowledge that I only had a couple months left. If there was no end in sight, and I knew that it would just continue straight through to the following year, I probably would have burned out (as would many others).

Finally, many (most?) students work during the summer. Some have to in order to help save money for college. I imagine that everyone here supporting this year-round schooling has enough money that it isn't an issue. However, I know quite a few people whose summer jobs throughout high school accounted for a portion of their college educations. Even if they don't need the money, taking a summer job is the most valuable way of obtaining work experience. Very little transfers over from the classroom to work. Holding a summer job (regardless of the type) helps students to learn how to interact better with people. At the elementary and middle school levels, free time during the school year is becoming increasingly occupied with extracurriculars - which is certainly not a bad thing - but studies have shown they also need time for unstructured recreation. Summer vacation offers that.