I'm half having the same issue. I have two school-aged kids. One changes into the uniform, but the other either wears pajamas, or if I manually change her into everyday clothes she wears those. It's always the same one that wears the uniform, and the same one that won't. Haven't tried the age up/down thing, though.
It happens to me too. No mods/cc in my game. Started a fresh save, and play with an existing family in Mt. Komorebi.
Not only they don't change into their schooluniform (when wearing ANY type of clothing, so not only pj's), but the whole family (in fact all the Sims in all the worlds) never change to/from hot or cold weather outfits going outside or back inside. This problem already exists since Seasons.
Maybe the not changing to schooluniforms belongs to the same problem?
