PLOT: witness to a murder

A group of high schoolers in a big city pass the time by playing a sort of game. They sit outside and “people watch”, making up background stories about who they see. Whomever’s story is the closest to the real thing wins. So a lot of times, they follow the people to see where their real-life takes them. One time, they follow the wrong person and end up witnessing a murder. Now, the bad guy’s are trying to kill them before they go to the police.