I am a graduate assistant responsible for teaching a lab. I am in the United States. I received an email from a student explaining a situation that we have to act on. He sent the email to both me and my supervisor (included in the To section). I do have the authority to act on it usually so that's not the issue.
He addressed it this way: Dear <my name> and Dr <supervisor>, Do I go ahead and respond directly to the student? Do I include the supervisor in the "To" the way the student sent it or do I move it the Cc section? Or do I allow the supervisor to respond as he has the higher authority here? He teaches the lecture, I teach the lab.