I went to school in France, where "redoublement" (staying down a year) meant that by year 11, there could be as much as a four year gap between the youngest and oldest in a year group (some people also got bumped up).
We never noticed who was a year or two older- everybody just rubbed along companionably. One girl in my upper high school year (year 12 afair) had taken a year out to have a baby then come back. Her year older than us was unremarkable, as there was also a boy who'd been asked to take a year out to sort out a drug problem, alongside several who stayed down along the way.
By year 13 (terminale), there is hardly a class in France that does not contain a wide range of ages. Maybe some of the more "select" Paris schools (where average age per class is probably below average expected age) , but that is it.
So much for differentiation. They don't do it much. If you're not up to the curriculum, you get longer to do it.
There is a very active vocational education, that used to start at age 13 and mostly was not optional- ie if a child was directed into that section, there was little choice but to seek another school. So vocational courses were filled by default and also contained quite a few children with special need who were nonetheless in mainstream. Not sure how it works now. However, the various vocational courses were very demanding academically. My mum taught English in a Lycee professionel for years and it was proper A level stuff. They did a wide range of academic subjects (yes including a compulsory second language, even the special needs young people- remember these were the ones deemed a little too dim to pursue general studies at 13, 14 or 15!)
Baccalaureat pass rate was about 75% back in the 80s. (but see proviso above re vocational courses, that hived off the lower achievers early in secondary school). But if you failed, you could retake (as many times as you wanted back then, not sure now). A friend of mine retook 6 times (he never passed
, and gave up trying at 24 as his classmates were getting younger and younger.