1 - The only way to do this entirely safely is to create a new sim who is a teen but is based on a CAS saved sim made out of the YA one. That's of course not the same thing as aging a sim down, it will be an entirely new sim who looks like the original but with no accumulated skills or experiences and no relationships to anyone else in the game until such relationships are established in other ways. But the game really doesn't like it when sims are aged down below the YA stage, all kinds of glitches can arise if you force this to happen through CAS for example on an existing older sim.
2 - Maybe I've just been really fortunate all this time, though I haven't played in SV for many years, but I've never had any trouble with any of the science centers I've placed and gone on to use for the location of a custom private school by way of the Careers School mod. Even science center rabbithole rugs placed in the basement of an overly simple shack looking structure have worked out fine, provided proper access is given to the rabbithole rug and there is enough space around it for sims to get in and out as required.
If you don't want to go through the steps of designing your own, perhaps try just about any of the pre-made structures from other worlds as available through Build/Buy?
