An assistant principal at the Bronx school where Abel Cedeno fatally stabbed another classmate in 2017 failed to act when the boy’s mom warned he was bringing knives to school and should be fired, a damning probe by the city’s schools watchdog found.

Cedeno’s mother phoned teacher Justin Keating at the now-shuttered Urban Assembly School for Wildlife Conservation in 2014 to warn him that her son was carrying a knife in his school backpack, the report, published Thursday by the Special Commissioner of Investigation for the New York City School District, found.

But when told by Keating about Cedeno’s violent tendencies, Assistant Principal Caridad Caro didn’t notify the school principal or anyone else, and didn’t log the incident into the DOE’s online reporting system, the SCI report says.

Caro told SCI investigators she searched Cedeno’s backpack in 2014 — two and a half years before McCree’s death — but didn’t find anything, so didn’t bother doing anything else.

The investigation found Keating, however, warned the entire school staff in an internal message, saying “this needs to be addressed immediately.”

Special Commissioner Anastasia Coleman recommended Caro be fired by the department. A DOE spokesman said Caro was reassigned to a central office in Aug. 2018 when the department first learned of the allegations, and DOE will now “start the termination process,”

The school was shuttered in June 2018 following the attack.

Cedeno, now 20, was sentenced to 14 years in prison Tuesday after for fatally stabbing classmate Matthew McCree after what he claimed was years of homophobic bullying.

McCree’s mother Louna Dennis — who is suing the Department of Education in Bronx Supreme Court over her son’s death — said she was “disgusted” by the contents of the report.

“I pray to God that after all this is said and done that the Board of Education starts doing what they need to do and starts taking the proper protocols to protect our children in the system,” she told The Post.

“I don’t want this to happen to anyone else. This is a nightmare. I’m living a nightmare.”