Robert McMillan:

A black Volvo rolled slowly down a quiet street while passengers armed with waterguns scanned for their target. Then at two minutes before 5 p.m., high-school senior Gabriel Pardini emerged from a YMCA.

From his middle seat, Graham Iwanchuk raised his Nerf Super Soaker XP35 and fired through an open window. Pardini never stood a chance. He was out of the game.

They were playing Senior Assassin, a citywide game of water-pistol tag that’s become a rite of passage for students across the country—and a major headache for school administrators.

Variations of the game date back to the 1980s, but today it is played with an app called Splashin. Each week the app gives every five-person team a squad to target. Their assignment: take them out with a splash of water. Iwanchuk’s team, the Last Troll, is part of a game that involves 250 other seniors at their school. Every hit is recorded on video and uploaded on the app, where they are widely shared and dissected.

Teenagers sneak into bedrooms, get younger siblings to knock on doors while they hide in the bushes and enlist spies to give up a target’s location. Players can use pretty much anything that gets you wet, from a water balloon to a hose. But water pistols are the weapon of choice.