It varies considerably. Most public schools break have either a middle school or junior high school (grades 6-8 or 7-9) between primary and secondary grades. But most Catholic parochial schools run to 8th grade. A lot of private schools run from 6th to 12th grade on the same campus, some 3rd to 12th.
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.)
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.)