Why do Conan and the Detective Boys call Ai "Haibara-san"?
To expand my comment, it's because she's a transfer student and always act mature



Ayumi tried to call her Ai-cahn several times but too afraid to do so



But in the end, she was able to call he Ai-chan, but when the other called her that, she refused



Pictures taken from Case Closed Chapter 398 and 400