[Always *]

In college or university, it's totally normal, I know that much. A lot of people don't start university at 18 or they might have changed majors or need an elective (e.g. a 2nd year elective but being in 3rd year). Personally, I did this tons in university because I didn't necessarily take my courses in "order", with the exception of core courses because this is how my program was; I also switch majors and didn't take a full course load for quite a while because of my learning disability. I think it is weirder for 1st years who might not understand why an upper year student is in their course because they're not so used to people backtracking, this is especially true for 18-19 year old students who don't really "get" yet that some people might start university in their 20's or later....

In high school, I know where I went that sometimes people would back track, lets say they needed a math class that would be on the university trajectory because we had courses (e.g. math science english, french etc) at different levels. So there was academic (university stream) applied (college stream) and workplace (literally not able to get into post secondary school but able to get through high school). In grade 11/12 it was divided further just for the college/university streams (so there were 4 levels for some courses like math). Any ways, someone might've started in the applied stream but realized it was too easy or realized they do want to go to university etc. and need to take a course (e.g. maybe they're in grade 11 and need to take a grade 10 math class at a higher level to qualify for the grade 11 math class or something). I never thought it was a big deal, especially with electives.

Personally, I always tried to avoid spending time in grade 9's or college freshmen courses, especially when I was a junior or senior. I might've had friends who were freshmen, but there is a huge difference in maturity between freshmen and juniors/seniors and I never wanted to be in courses where people would goof off, work less-well, etc. but that's just me. It's still ok to do, I had to because it was unavoidable.