...somehow I don't believe that an asshole cop like that will play totally by the rules and not search the car if you tell him you don't want him to.
I mean in the end if he finds weed you can tell the judge whatever you want and he will believe the cop anyways. If he doesn't find anything and you sue him for searching your car, he will also be able to wind himself out somehow, because he's a cop and he's always the most believable person in court.
...and by the way: imagine those teenagers were black instead of white and wearing hip-hop clothes - the cop wouldn't probably even have asked if he could search the car, he would have just done it.