Sims/Pets age up multiple times in a row

Some sims tend to double age aka skip one age stage when aging up. My sim was an infant and aged up straight to be a kid. I reversed this in CAS so the sim was a toddler now. He ended up getting school homework in his inventory, but I didn't think too much about it.

Then, my sim aged up to a kid. He started school with the grade of F (in Finnish version grade 4, so I think grade F is the right translation here?). So the game logic seems to think that the sim has been skipping school while he was still a toddler.