Justin Bieber -- Beer Pong Time

He might not be enrolled in school ... but Justin Bieber is definitely getting a taste of the college experience ... 'cause we got a photo of the Biebs playing an intense game of BEER PONG.

Even though he's underage, we know Bieber pounded some beer during the game ... just like every other underage college kid across the country.

The question -- which country?

One source connected with Bieber -- who was not there -- says the game went down on Canadian soil ... or in Georgia (he travels a lot, it's hard to keep track). Either way, the source says it happened a year and a half ago, which would have made Bieber 17-years-old ... and too young to legally booze in either country.

Another source -- who was there -- swears it happened in Alabama back in June ... which still makes Biebs too young to booze.

Still, one question remains -- why the hell is he wearing a backpack?