@Darth_Noox
Basically in Japan, it's considered rude if you call people by their first name. It's because first name call is reserved for family, or anyone that very close to you.
It's simply like how American use last name for polite calling of people
Somehow, if teen call other with different gender with first name, it might considered as they are either dating or having now each other since long.
In this case, Chiyo feels like that they are really dating for calling him with first name.