Jo, the US school schedule has roots in our agrarian past. I think that school traditionally began early so that children could be home in time to help with chores, and recessed for the summer so that children could work in the fields. The fact that 90+% of current students have never even been on a farm hasn't stopped the inertia of that schedule.
I think that the inertia is compounded by the work ethic. You are right; people who take what vacation time they are given are regarded as slackers. Also, however, there is the problem of trying to make up the work if you take the days off. The last time I took a week off - 1995? - I spent such long hours before and after that week doing the work I couldn't do while I was gone, I told myself I would never do that again. It's just too painful.
There has been in recent years a slow alteration of the traditional school schedule. The school year is creeping later and later into June, and beginning in late August rather than in September, and parents are beginning to grumble about the early hours (when I was in school, I got up at 5:45). Suggestions to have school year around are receiving more favorable attention, in no small part because not having the extended recess would alleviate the learning atrophy which occurs in children and requires them to spend a significant part of each new school year reviewing the prior material and rehoning their learning skills. There are three major obstacles to year-round school schedules: in some areas of the country, the lack of air conditioning in school buildings (never installed because the buildings were closed during the hot months) would make summer sessions unbearable; teachers - at least, teacher unions, which are extremely strong in Michigan - don't want to lose their summers; and the tourism industry does not want the end of summer vacations because of the adverse impact on vacation travel (in Michigan, a few years ago the tourism industry tried to get a law passed outlawing the beginning of school before Labor Day, which is the first Monday in September; the compromise result was a prohibition against holding school on the Friday before Labor Day).
Always, there are vested interests in the status quo which slow change, even when the interests are not really pertinent to the issue at hand. Sigh.