I grew up (in Ireland) with single-sex schooling until age 12-3 (primary) and mixed secondary. That has its own potential pitfalls (eg bullying, more playground fights) but I think there are benefits too. Also, in primary school it's the same teacher all day for a full year (or two) at a time, which makes it a lot easier for the teacher to know each pupil and manage the classroom appropriately. I'm unsure why America…
I suspect that the break is because in the middle years it becomes harder to find a teacher who can do everything--reading, history, math--adequately. A lot of what is called "science" in American primary schools I would call "environmental civics"--recycling is good; don't litter. Then in 7th or 8th grades one can actually encounter chemistry labs.
(On the other hand, in my 7th (8th?) grade science class a big chunk of time was given up to the evils of drugs; I didn't particularly disagree, but bilged the class because I found it tedious to cull clippings about overdoses or LSD flashback. And I have no idea what I might have learned in the "science" lessons of the earlier grades--how many planets, maybe.)