SEASIDE, Calif. — A teacher at a Northern California high school accidentally fired his gun inside a classroom, causing minor injuries to three students, but kept teaching while the students sat there, the mother of one of the students said Wednesday.

Dennis Alexander, a reserve police officer, was pointing the gun at the ceiling Tuesday to make sure it was not loaded when it discharged inside his classroom at Seaside High School in the coastal community of Seaside, police said.

Seaside City Council member Dennis Alexander speaks on Oct. 4, 2010, during an election forum in Seaside, Calif. Alexander, a teacher in Northern California accidentally fired his gun in a classroom, injuring three students, police said. Alexander, who is also a reserve police officer, was pointing the gun at the ceiling to make sure it was not loaded when the weapon discharged Tuesday, March 13, inside his classroom at Seaside High School, KSBW-TV reported. The Seaside Police Department said no one suffered "serious injuries." Monterey County Herald file photo via AP

Police said no one suffered serious injuries.

Alexander was not authorized to have a gun on campus, Monterey Peninsula Unified School District spokeswoman Marci McFadden said.

District policy says only school resource officers — active law enforcement personnel employed as armed security at schools — can carry guns on campuses.

Authorities were investigating why Alexander had the gun and if officials were aware that he intended to bring it to school, McFadden said.

Fermin Gonzales, 17, was injured when bullet fragments lodged in his neck, his mother Crystal Gonzales, told The Associated Press.

“I’m still really upset no one called a nurse or a paramedic to come check on the students,” Gonzales said. “They just sat there until the bell rang.”

The accidental shooting came amid a national debate over whether to arm teachers in the aftermath of a mass school shooting at Marjory Stoneman Douglas High School in Parkland, Florida, that killed 17 students and staff members.

On Wednesday, thousands of high school students walked out of classrooms across the nation to protest gun violence in schools.

A law that took effect in California on Jan. 1 halted the ability of school districts to allow non-security employees to carry guns on campus.

Gonzales said the incident in Seaside happened Tuesday at 10:30 a.m. and that she did not hear about it until her son called her hours later when the class ended and he went to a relative’s home.

Alexander was teaching a gun safety lesson in an administration of justice class and was about to show the students how to disarm someone, Gonzales said.

She said no officials contacted parents to let them know what happened and that she was shocked when her son returned home with blood on his shirt and bullet fragments in his neck. As the boy’s parents rushed him to a hospital for X-rays, she said the school’s principal called her cellphone to apologize.

The teen is fine, though he’s still shaken up and stayed home Wednesday, his mother said.

Gonzales said police didn’t arrive at the school to investigate until three hours later and the family filed a police report.

Alexander was placed on administrative leave from his teaching job and he was also placed on administrative leave at the Sand City Police Department, police and school officials said. Efforts to reach Alexander were not immediately successful.

Then school district sent a letter to parents saying its human resources department, the high school administrators and the Seaside Police Department “immediately began investigating the incident, including interviewing students in the class.”

It said counseling was made available to students and that it could not release any other details “due to the nature of this personnel incident.”

Associated Press Writer Jocelyn Gecker contributed to this report.