Courtesy Jeff Taylor Lebanon High School senior Jeff Taylor, 18, with the water gun he uses to play "Assassin," a game that has been banned at New York City's Hunter College High School and others across the country.

An elite New York City high school is warning seniors it could ban them from prom or graduation — or even snitch to college admission officers — if they're caught playing a popular toy-gun game in or near the school building.

The game is called "Assassin" or "Killer," and it's played at schools across the country, usually in May after exams end. Rules vary, but it generally involves students stalking and shooting human targets with water pistols, Nerf darts or plastic disks until only one remains.

Players say it's a fun way to blow off steam, but some school administrators and police officials fear it could turn deadly serious.

"Parents and students should know that we consider this a dangerous game and prohibit playing it on campus," Hunter College High School Principal Tony Fisher wrote in an email to parents last week.

"You should be aware that any students found playing the game within the school or in the immediate vicinity of the building will receive disciplinary consequences."

Fisher declined comment to NBC News but his email details the potential penalties: banning a player from senior events, suspending them, or reporting the incident to colleges if it's not their first serious transgression.

"At least one Senior has been excluded from Prom as a consequence of getting caught playing Killer for each of the last five years,” Fisher wrote.

His concern, echoed by other administrators who have cracked down on the game in recent years, is that the popular diversion is riskier than it seems on the surface.

A student being pursued by an "assassin" could dart into traffic, or a water pistol could be mistaken for a real gun. Teens could be tempted to break laws while they hunt their prey.

Police in Stoughton, Mass., said the dangers aren't purely hypothetical.

"Some of them really do take it too far," said Deputy Chief Robert Devine, recalling a scary incident two years ago.

"It was six in the morning and this kid was proned out [laying on the ground], wearing camouflage, behind a fence, waiting for his target to walk by. A neighbor saw it and the water gun looked like a real firearm and before you knew it, this kid had two officers pointing firearms at him," Devine said.

"And it wasn't the first time we've had calls like that," said Devine, who worked with the local high school to discourage kids from playing the game.

A game in West Jefferson, Pa., was squelched this year after police got reports of teens in high-speed, reckless chases, according to the Pittsburgh Tribune-Review. A Hillsborough, N.J., school was locked down in 2011 after a report of someone pointing a gun out a car window; it was later determined to be a player with a water pistol, the Hillsborough Patch reported.

The NYPD told Hunter that the students' annual "Killer" session overlaps with a gang-initiation period in the city, and that gang members could paint their real guns to look like toys — creating a confusing, dangerous situation for police, Fisher's email said.

At Lebanon High School in New Hampshire, a round of "Senior Assassin" is in its third week after starting with scores of players.

Senior Jeff Taylor, 18, who made it to the semifinals, said he doesn't see anything wrong with it.

"I just have fun doing it," he said. "It's a friendly rivalry and I'm a competitive person."

He said no one could mistake his water weapon for a real gun: "I'm using a super soaker. It's bright orange, blue and green."

Nikayla Cartier, 18, who also attends Lebanon High, said she can "totally understand" why some grown-ups are aghast, "but in all honesty, there's never really been a serious problem."

Cartier, who was eliminated on the first day when someone ambushed her at home, admitted some classmates go overboard. One staked out a spot on a friend's roof like a sniper, waiting for his target to walk by.

"It's extremely stressful because you're watching your back 24-7," she said. "But it’s a good kind of stressful."