Answer:
¿Cómo estás?
Step-by-step explanation:
¿Cómo estás? = How are you (informal) To start a conversation
Adiós. = Goodbye
¿Cómo está usted? = How are you (formal)
Estoy mal. = Im not feeling well
Two young schoolgirls will likely use informal language with each other (tu form) so they would probably start a converstation with each other using "¿Cómo estás?"