Teen shutdown is often indistinguishable from dismissiveness, but underneath, the nervous system has collapsed inward. The teen who will not look at you, will not speak, leaves the room: they are not being rude. They are regulating the only way available to them right now. Trying to force engagement when someone is in shutdown is neurologically counterproductive and damages trust.
I am not going to push. I just want you to know I love you and I am here when you are ready.