A terrifying “mass stabbing” allegedly planned for a Fresno high school on Sept. 11 was thwarted after a student spoke up and alerted authorities to the chilling plot.

The suspect who allegedly planned to carry out a mass stabbing at Duncan Polytechnical High School was arrested Wednesday, the Fresno Police Department said.

Cops said they were notified after a friend of the suspect sent a message to others warning of the horrifying plot at the high school, insisting it was real and to not go to school on that day, according to KSEE 24.

“There is going to be a mass stabbing at Duncan High School on September 11th, this is not a joke, or a hoax,” the message read. 

“My friend who goes to Duncan is planning an attack on Duncan for unknown reasons other than how he doesn’t like people.”

“PLEASE believe me, do not go to school l[on] September 11th if you go to Duncan,” it added.

The message traveled to another student and eventually to a parent and then to the Fresno police.

Fresno Unified School District said this “incident stems from a threat made in a chat on social media.”

“The Fresno Police Department and school resource officer (SRO) were able to identify the student involved. The individual will face appropriate disciplinary action.”

Fresno PD told The Post “a minor juvenile was booked into custody for charges related to the investigation” and said no other arrests are expected.

Reactions to the news on social media overwhelmingly credited the person who spoke up, leading to an arrest and preventing what could’ve been a tragedy on the 25th anniversary of the September 11th terror attacks.

“Big thank you to the student that did the right thing. Our granddaughters are freshmen there,” one person wrote.

“Kudos to that student! He needs a medal. This just proves if people would speak up when they hear about something like this it would be stopped before anyone gets hurt,” another person wrote.

Duncan Polytech is a public school part of Fresno unified.

The high school offers students interested in a career in Applied Technology or Health Sciences skills and competency in the field through “rigorous academics, real-life work experiences, and extracurricular activities,” according to the school’s website.