It's normal for teenagers to "wear the uniform" to fit in. They would rather die than look "weird." I recall that my parents couldn't tell me anything either.

At least hoodies offer some protection to the head and neck in cold/windy weather. So do those silly "gangsta" watch caps they wear in the dead heat of summer; or perhaps that fashion trend is over?

Seems to me the key might be to add some thin, discreet insulation underneath the whole works.

Adding a polyester athletic T-shirt under a cotton shirt adds both warmth and moisture management. A merino wool T-shirt would add enormous warmth over the body core. Yet it would be essentially invisible under the "uniform."

Adding some thin dress wool socks under cotton tube socks similarly adds both warmth and moisture management.

Maybe you could convince them to keep a super-thin headband in the pocket of their hoody, even if they don't usually wear it.

These little improvements add up a lot.

Worth a try, anyway.