I usually give my teen girls ponytails or the hairstyle Morgan Fyres wears and shy away from the shorter, choppy-layered hairs because (in my opinion) they make them look older. Also some of the braided hairs look a bit youthful to me. I dress them in jeans, hoodies, graphic tees, no low necklines or too much skin showing. For their formal wear I dress them in the shorter, ruffled party dress with the flats that came with City Living or anything else with little to no heel. I also add braces to their teeth. For males, I try to keep their hairstyles more playful than professional (I guess you could say), and add acne (sometimes). Also lots of graphic tees and the knock-off converse for both genders. Shorts and flip flops work too. You could also dress the girls somewhat Cassandra Goth to make it look like a school uniform as well. I've done that too.
Edit: The hairstyle on my sim in my profile pic seems more young adult to me than teen but I have used it on some teens. It all really depends on their personality.
Edit (again): As for make-up, I usually use neutrals to give the barely-there look, I guess you could say.
