I mean, Yufu and her boyf may have an open relationship or something.
and while the message of the story can be interpreted as a "do bad things and have fun", I think I take it as a much... lighter story? and can really enjoy how silly it is.
The representative is obviously not really... having a good time at school? She studies but that's really all of her life and she's extremely aware of her deficits in other areas. The girl doesn't have friends at school and is lonely a lot of the time. She knows the rules but doesn't really fully understand the full meaning behind them? People with a stronger moral compass would refuse to drink and would return the shirt anyway. While I don't approve of how it was done, I do appreciate the fact that she's getting all sorts of new experiences. It makes her more worldy and able to relate to people she normally wouldn't be able to (plus ridiculous stories). I doubt she's the type to ever go overboard but learning your boundaries is a very important part of growing up and deciding what kind of person you want to be (wow I sound old). But overall it's just a fun story :P