Question re: 4th ammendment rights...

[ddss6_99]

So a bunch of people including myself, all of whom are under 21 went to a party on at my college tonight. To get to the party we need to take a trail through the back of campus. Everyone takes the trail because it's the quickest way to get to the off campus appartments, so it's usually filled with 200+ college students at any given time. So as we're walking down the trail, a cop who is just chilling on the side of the trail shines his flashlight at my friend and asks to see what's in his backpack. He reveals his backpack which has approx 30-40 beers in it. So this is later used as probably cause to search his person, which there was less than 4oz. of marijuana on. Question is this, since we were'nt breaking the law or being suspicious in any way, we were just walking as were hundreds of other kids, did the cop have the right to just pull someone aside and search their belongings. I realize that he opened his bag for them
, which probably wasn't the best idea, but if he were to say no when asked to open his bag, would the police have been able to do anything about it or would that have been the end of it?