American education is mostly run at the state level, so it changes depending on where you're attending. In Illinois, attendance is mandatory until 16 years of age, and junior high tends to be 6th-8th grade (11-13 years old, give or take by month of birth), but I know that changed more than a few times just in my parents' lifetimes, though I think it's been stable for more than a few decades, now. But woe to the classes who were about to be the top of the heap at their school...only to have that grade re-assigned to being youngest at the next "high", junior or "senior" high school (which no one EVER calls "senior high", it's just "junior high" and then "high school".)

And yeah, the sizes depend on the local population size. In the west 'burbs of Chicago, my freshman high school class had over a thousand students (less than 350 when I graduated, eep.) But driving to our folks' house in semi-rural Indiana, we pass a sign for the "Tri-County High School", meaning it serves at least a dozen towns. Younger Brother still boggles at it, heh. *g*