It’s a sign of the times.

The classic schoolhouse prank of slapping a “kick me” sign on a classmate’s back is no joke at one Upper East Side school, where the city’s zero-tolerance anti-bullying policy was strictly enforced against a 9-year-old boy.

The fourth-grader was spotted sticking a yellow Post-it inviting “Kick me, please” on another little boy’s backside on Jan. 25.

But instead of getting banished to the corner for a timeout, the PS 158 prankster was bounced from the classroom for two days, The Post has learned.

The boy’s mom fumed that the old-school gag was just “playing around” — but a parent who asked to remain anonymous called the Post-it “maliciousness — it’s not kid’s play.”

The stunning response to the taunt was a foregone conclusion, given that city schools formally banned bullying in 2008.

“School authorities might not have the leeway to do things differently,” said Richard Gallagher, the director of the Parenting Institute at NYU’s Child Studies Center.

“We have to be very careful about zero tolerance and how it gets implemented,” he said. “Teasing behaviors are pretty typical. But the boundary between teasing and bullying is different.”

Upper East Side psychologist Dr. Joshua Rosenthal said the Columbine massacre in 1999 scared educators.

“It was a wake-up call as to what bullying can lead to . . . in extreme cases,” he said.

The little boy’s behavior triggered a stern warning to the boy’s parents by school Principal Darryl Alhadeff.

“This incident is in violation of the Discipline Code and is classified as ‘infraction A37’ — engaging in bullying behavior — and will result in a Principal’s Suspension for a period of two days,” he wrote.

A Department of Education spokeswoman said the kick-me caper was “just the latest incident between the students. The principal followed procedure, held parent conferences and offered counseling.”

Crime of the century? Some of the PS 158 kids think not.

“[He] was probably teasing” his victim, one pug-nosed fourth-grader said in the Yorkville schoolyard yesterday.

“[The boy] either teases you or he tells you he’s going to start hitting you, but never does.”

Additional reporting by Yoav Gonen

jamie.schram@nypost.com

