A Missouri high school student was harshly punished after she recorded her white geometry teacher repeatedly uttering the N-word last week.

The footage led to the suspension and resignation of the unidentified teacher — and left the 15-year-old sophomore with a three-day suspension for inappropriately using electronic devices.

The strict discipline dolled out by Glendale High School administrators in Springfield left the girl’s mother “genuinely shocked.”

“To punish someone in this situation who does the right thing, it’s absurd,” said Kate Wellborn, who said her daughter Mary Walton used her phone to record the teacher saying the slur.

A 56-second video recorded by Walton and shared with KYTV-TV showed that the teacher used the term twice, while debating the semantics and appropriateness of its usage with a student.

Walton said the teacher, who is white, had uttered it a total of six times — including four times before she pressed record.

“I don’t like the word, at all,” the teacher was heard saying. “I don’t know. It feels like when a black person is using it towards another black person, it’s the same. How is it not still a derogatory word?

“Is the word n— not allowed?,” he then asked, in response to an inaudible student inquiry.

“I’m just saying right now, as a teacher, if you want to keep your job … This isn’t a threat from me,” a student replied.

“I’m not calling anyone a n— ,” the teacher countered. “I can say the word.”

Walton shared the footage with school officials and did not post it on social media. The local outlet blurred his face and the faces of other students in the shot.

Principal Josh Groves told employees and families in the district that the language the teacher used was inappropriate and unprofessional. The teacher, who had worked for the district since 2008, was placed on administrative leave. He resigned earlier this week.

Even as the school used the evidence captured by Walton to discipline him, they did not use any discretion when it came to throwing the book at her.

A spokesman for the district called Walton’s actions “unacceptable,” noting the student handbook considers recording teachers and students a “violation of privacy.”

The statement went on to say that students should “follow the appropriate steps for reporting” any classroom behavior that concerns them — and not record it.

Natalie Hull, the family’s attorney, said that it was unreasonable to ask a 15-year-old to know the “proper channels” for recording such incidents.

“Frankly, many of them don’t know if they’ll be believed,” Hull said. “It makes sense that they would feel the need to capture hard evidence and indisputable evidence.”

The district denied Hull’s request to let Walton return to school after serving the first day of her suspension.

Other prominent supporters have also asked the district to give Walton some leeway.

“The student says she was recording the teacher’s alleged racist remarks for the express purpose of making a record of the incident should the events in the classroom at that moment come into dispute,” Dan Shelley, president and CEO of the Radio Television Digital News Association wrote in a letter to Springfield Superintendent Grenita Lathan.

“In our opinion, that makes her a lawful whistleblower, not a delinquent. She should be congratulated, not punished.”

The pleas had fallen on deaf ears, according to Walton, who said the school has refused to apologize and would not consider taking the infraction off her daughter’s permanent record.

With Post wires