Why don't schools in the Sims have actual grades, instead of just elementary school and high school?

That would require chopping up the lifestages of "child" and "teen" into a lot of little subgenres, which would be hard if not impossible, to mesh with our being able to adjust Sim lifespans AND adjust the length of individual seasons. It's already pretty awkward with situations in which a Sim can be a Young Adult in spring, and an Elder by winter. The period of Sim-relative time in which a Sim is a child and a teen doesn't translate cleanly to the number of seasons that pass, so designating which exact year of age (and corresponding grade level in school if we go by age and bypass the idea of advanced or repeated grade levels) would be difficult.

I think to keep them from being Young Adults while in 6th grade, we'd have to see a grade level change per day or something.