Here's why:
* Subject: The subject of a sentence is the person or thing that performs the action of the verb.
* Verb: The verb in this sentence is "are".
* Who is doing the action of being a new student? The "you" is the one being asked if they are a new student.
Even though the sentence is a bit informal and grammatically incorrect (it should be "Hi, my name is Paul. Are you a new student?"), the subject is still "you".