answer: Mr. Rogers asks Mary Lin to sit behind Brad and says the first period will begin when the bell rings.
documents: #Person1#: Mr. : Are you Mary Lin?
  #Person2#: Yes.
  #Person1#: Mr. : I'm Mr. Rogers, your homeroom teacher.
  #Person2#: Where should I sit?
  #Person1#: Mr. : Why don't you sit behind Brad?
  #Person2#: Who's Brad?
  #Person1#: Mr. : He's that guy in the blue shirt.
  #Person2#: When does the first period begin, by the way?
  #Person1#: Mr. : As soon as the bell rings after the morning announcement.