^This

I agree with @PuppyPotter27 , is just like soda

But the professor don't afraid their students drunked?

No because it doesn't affect them

Firewhisky, on the other hand, IS an alcoholic beverage and students under 17 may not drink it.

I doubt a human could get drunk form Butterbeer (unless they drank like a dozen bottles in a day or something), Winky had to drink six before she got drunk and it's much stronger for house-elves

You need at least to be six year so you can drink it?

So I just looked at the wiki article and it said it had very lower alcoholic content, but it's still fine. Maybe read that.

You need to be at least 17 (of age) to drink firewhiskey, just like with alcoholic beverages in the muggle world

I just looked it up- most soda's have a very low alcoholic content as well, so it's very similar to a soda