A “heroic” NYPD school safety agent was slashed inside a Bronx elementary school Thursday morning — as he stepped in to protect a female guidance counselor whose irate ex-boyfriend showed up to attack her, cops said. 

The 23-year-old suspect arrived at P.S. 69 Journey Prep School on Thieriot Avenue in Clason Point around 7:40 a.m. to confront his ex, a 36-year-old guidance counselor, authorities said. 

The 57-year-old on-duty school safety agent stepped in to shield her — and ended up getting slashed in the back of his left ear, cops said. 

“[The suspect] attempted to assault a teacher,” Mayor Eric Adams said during an unrelated briefing about the city’s Gifted and Talented program. “A heroic act by a school safety agent prevented it from taking place.”

A 40-year-old male teacher was also slashed on the left arm, police said. 

The guidance counselor suffered a bump on the back of her head, cops said. 

The school safety agent and the counselor were taken to Jacobi Medical Center in stable condition, according to police. 

The suspect was taken into custody with charges pending, police said. 

“That individual will be brought to justice,” Adams said. “It is unacceptable. Violence in our school will not be accepted, and I want to thank our school agents, safety agents all over the city.”

The incident happened before students arrived Thursday morning, and no children witnessed the attack, according to the city’s Department of Education.

“Violence has absolutely no place in our schools, and this senseless act against an essential member of our school community is unacceptable,” Schools Chancellor David C. Banks said in a statement. “Our incredible educators and School Safety Agents work every day to keep our children safe, and I am grateful for their steadfast dedication to our schools. We are offering supports to this school community.”

In a statement, Michael Mulgrew, president of the United Federation of Teachers, said the incident “highlights why we need to have a full contingent of school safety agents.”

“We are down at least 2,000 safety agents from where we should be,” Mulgrew said. “Today, we are grateful to the teacher and school safety agent at PS 69 for stepping in to protect their colleague and thankful that no one was more seriously injured.”

“The de-fúnders won today and a school safety agent was stabbed,” added Gregory Floyd, the president of Local 237 Teamsters. “We need more school safety agents now. What will it take? Death? Mayor Adams: hire more school agents now.”

Additional reporting by Bernadette Hogan and Carla Bamberger