It's Beanish custom to welcome the guests first, and then the colleagues.
Amtoo is dropping the rope ladder for Ms. Frizzle. He welcomes her using the formal greeting style, expressing his esteem. By responding in standard greeting style, Ms. Frizzle invites him to be less formal.
BrokenBean is a friend of Amtoo. He calls him by name, with a very informal greeting.
La Petite is listening attentively, trying to grasp these details.