How does your school system address the problem of students who are more advanced or slower than their grade-level indicates?
First, for the really slow ones, there's always the possibility of being held back a year, possibly repeatedly (this might not be as common as it used to, they don't like holding back kids anymore, it's bad for their self-esteem or some other BS). It is possible to skip grades (usually never more than once) in primary school (except for grade 6 I guess, as it is the last one before secondary school, and requires passing "ministry" exams, that is exams made and mandated by the government, which are uniform across the province). In secondary I don't recall seeing any skipping though it might not be impossible.
There may be one optional class in the first 3 years of secondary school, sometimes only offered to smarter kids, e.g. I took 3 years of latin, but the first two were "extra credits" for the top 40%, while the last one was just one option among many, though it was the only option with a prerequisite.
It's only in the last two years of high school (sec 4 and 5) that some real variety appears in the curriculum. There are "core" classes that are necessary to obtain a diploma, such as sec 4 history and English, so those who don't pass these courses the first time around (including supplemental exams in the summer) have to take them again the next year (without otherwise being held back a year).
It's in Sec 4 when "streaming" begins in math and sciences, "strong" for top 40% vs "weak" for the rest, at least back in my day, though there has been more possibilities since (usually something in between in contents, or the "strong" stream but with more time alloted to it). Sec 5 math is required for all but again, there a different types, the "strong" kind is a co-requisite for chemistry and physics classes and together all three classes are required for the science pre-university program in cégep. Those who are not going into science take other electives instead (such as history, journalism, arts, computer science, languages, etc).
Still, many schools offer sport/arts-studies programs where half the day is spent on academics (condensed, electives often reduced to what is required for science in cégep as they will be sufficient for any pre-university program), the other half on sport (or arts), often in seperate institutions (such as, say, a private music school). As expected, students in those programs are required to keep their grades up above a certain average (GPA). Other similar "advanced" programs include English immersion (in the French school boards, I think there are now third language programs as well) and IB. None of those approaches provide faster advancement toward cégep (and/or university), but their "enhanced" curricula are quite attractive to recruiters in competitive programs.