If you can't drink alcohol, why not drink coke at a club? It's also a dance club, and one the characters of P3 frequented despite not being able to drink.
And they (actually, only Yukiko and Rise) acted drunk because, as I've read, alcohol is mostly a mental effect. I mean that in the sense that if you think you're getting drunk, you'll act it. They were thinking they were actually drinking (though Naoto confirmed they weren't)
And would you hit on a group of girls when someone like Kanji is with them?
Teclo said: It's only just sunk in how stupid that scene is, with them sitting in a club in their school uniforms and drinking soft drinks. You don't go to a club and drink Coke. It was very odd how they also started acting drunk for no reason. I mean, was it just changed to soft drinks in the US version? You see 16-year-olds drinking on TV all the time and everyone knows that no one waits until they're 18/21... So why the half-assed censorship job (if that's what it was)?
Also, if some schoolgirls went into any real club, they'd be getting hit on by all the men and half the women.
@BakaHoushi: Well I don't know about in Japan or in the US, but in the UK, if you're allowed into a club it means they think you're 18 which means you can also drink. I agree with you on the alcohol thing but it's more like, you'll get drunk anyway but you'll get MORE drunk if you're in a certain mood or situation.
Also, it was pointed out that there was no alcohol in the drinks. At all due to a few accidents way back when. The club hasn't been serving alcohol for a while as pointed out by Naoto
It's a way for the game to explore drunkenness in their underaged characters without having to deal with all the knee-jerk OMG THIS IS WRONG response that a lot of people have.
Besides, it's also tongue in cheek humor. Everyone knows that it's supposed to be alcohol.