I don't think people realize how much work it would take to make height sliders, in terms of animations. Too short, and a Sims hands would go straight through countertops when cooking, for example. More importantly, every animation that requires physical contact between sims would need to be redone in a lot of variations. It's not minimal anymore if you have to account for contact between the shortest possible sim and tallest possible sim. So it's changes to not only kissing animations, but hugging, taking selfies, taking care of toddlers, touching a pregnant woman's belly, vampires drinking, etc. I wouldn't get my hopes up if I were you. Maybe ask for it to be part of sims 5 base game, but even then it's still a lot of work.
As for preteens... I do kind of like the idea, but I fail to understand exactly what would be different about them, other than appearance. I feel like teens could simply be shorter and less adult like and it would pretty much work. They can already do everything you just said preteens do, and not much more. As for school, not every country has a school system with three different schools. No matter what they do, it's going to seem weird to some people.
