lots of cool ideas! Personally, I think what I'd most like would be a book/reading overhaul (preferably to be patched in, but I think it would extra work for some kind of school expansion). Things like:
The ability to buy bookshelves with different preset stocks of books (or to buy completely empty bookshelves if you want to build your book collection from scratch), so like you could buy a bookshelf with all level 1, level 2, or level three skill books, or have sets like a "science" bookshelf with all logic, rocket science and handiness books, or a "art" one with all the instruments and painting, or all the fantasy books, stuff like that. That would make building and visiting libraries a lot of more fun and interesting I think.
Reading books builds some kind of skill of kids. Toddlers can gain imagination, but reading does fuck all for kids (which is extra frustrating for the child mental aspiration, which needs to "read with an adult" which is nothing but a big ol' waste of time at this point). Personally, I think kids should build both creativity AND mental skill while reading, but have trouble with more difficult books which need an adult to read to them to gain the skill.
Sims should like/not like different books, based on their traits/interests. Bookworms can still love all books equally, but for everyone else I think there should be some diversity, and if a sim doesn't like a book it won't build their fun as fast and may cause a bored or even angry moodlet.
And lastly, sims should be able to buy books published by sims in the game. So, if my sim publishes a book, if I go another household, that book will now be available for purchase, and reading said book will have an effect based on its type and quality.
This may all be more appropriate for a "library" pack, but libraries are a big part of school in my experience, lol.
Other things I'd really like:
More after school activities, including sports and a way for kids to build the fitness and dance skill once they've maxed motor.
More options for controlling kids at school from home if you don't choose to follow them to a playable school, like that "better schools" mod where they can actually, like...learn skills and shit, lmao.
Some unlockable child aspirations, unlocked when they compete their first aspiration, that could give a jump start/advantage to certain careers or adult pursuits if completed. So, for example, completing the social aspiration would unlock, say, a "Class Clown" aspiration which would give you an extra extra boost to gaining the comedy skill when you age up, and some extra chance of success in the comedy branch of the entertainer career, and you could choose that one or one of the the other three base ones as a second aspiration to pursue.
School dance events, with an "ask to go to school dance" interaction for teens which would function like asking on a date but like...later, lol. Kinda like the promise ring/proposal action, which would be realized by planning the event. I'd like it to maybe be coordinated with the holidays from seasons and maybe even the festival type events from CL, but I know crossing over packs is not something the like to do, so...
And I think those or my top priorities. There's other things I'd like of course, but if we could just get these I'd be super happy lol.