I’m not a teenager but depends on her style, I styled silver trousers with a new grey pocket jumper from Zara.
But agree that band t shirt or cropped jumper is good. From any of her favourite shops.
If you want to get a more luxe version then ask what she wants. At that age I was dictated to by DM’s choices but as soon as I was 17 and had own money I traded up in what I wore.