I missed 3 months of 4th grade. When I came back, the teacher told my mom that I could not continue, because I'd missed 3 months of education. I'd have to finish out the year in 3rd grade.
My mom would have none of that, and demanded I be put back in 4th grade.
And so I was, and it was like I wasn't gone for a single day. The class had not advanced at all.
This was quite unlike university, where I didn't dare miss a single lecture.
I think there’s a significant difference between fourth grade and high school level math, especially the more advanced courses. I got the flu in 9th grade and missed a week of trigonometry. I was able to catch up and it wasn’t the end of the world, but it wasn’t trivial, there absolutely was a “catch up” period.
Agreed university is much harder though.