When the Pleasanton Police Department converged all available units toward an armed robbery call last January, the presumed crime was not the heist they were expecting. The weapons were Nerf guns. The robbers peeking into the windows were teenagers. And the surveillance of the house in question was part of an elaborate game played by high school seniors that police and school officials are hoping to disband this year. The annual game, known as Assassin, involves seniors from Foothill High School and Amador Valley High School firing foam darts from Nerf guns — often painted to look more like real guns — at a list of targets assigned to each player.