Teenagers aren't stupid. They're very intelligent. The difference is, their intelligence is invested in things that aren't as important to you (at least, not anymore). Namely, keeping up social conventions and presenting a specific persona that's expected of them.
Some kids are probably wearing it because it makes them look badass to other kids when they walk around in winter in summer clothing. Some are probably doing it simply because they forgot to put it on, a lot of them probably didn't look at the weather report and assumed it would be in the 50s, or they left it at school, they might hate wearing heavy clothing and coats (probably especially important for girls trying to get guys to look at them), others might be doing it to make a statement to their parents about how they don't need them, and the list goes on.