A Northern Californian teacher trained in firearm use has “accidentally” fired his gun inside a classroom, injuring three students.

Dennis 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 inside his classroom at Seaside High School in the coastal community of Seaside, police said.

Mr Alexander continued teaching the class while the injured teens were made to sit and wait for the bell, according to reports.

At least one student suffered injuries from bullet fragments to his neck.

Mr Alexander was teaching a gun safety lesson and was about to show the students how to disarm someone, Crystal Gonzales, the mother of injured boy told AP.

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

The student’s father, Fermin Gonzales, told KSBW 8 that the teacher told the class before pointing the gun at the ceiling that he was doing so to make sure his gun wasn’t loaded, something that can be determined visually.

It’s the craziest thing,” he said. “It could have been very bad.”

“He’s shaken up, but he’s going to be okay. I’m just pretty upset that no one told us anything and we had to call the police ourselves to report it.”

The teen, 17, was treated at a local hospital where the boy’s parents took him after he came with blood on his shirt and bullet fragments in his neck.

According to Monterey Peninsula Unified School District spokeswoman Marci McFadden, Mr Alexander was not authorised to carry a firearm on campus, AP reported.

It comes after President Donald Trump’s push to arm school teachers to prevent school shootings.

“If you had a teacher with – who was adept at firearms, they could very well end the attack very quickly,” Mr Trump said during a listening session with six students who survived the school shooting last month.

He reiterated his stance on Twitter, saying “attacks would end” if teachers were armed.

“A ‘gun free’ school is a magnet for bad people,” Mr Trump tweeted in February. “ATTACKS WOULD END!”

The news coincides with a Florida judge entering a not-guilty plea for Nikolas Cruz, who faces a possible death penalty in the high school massacre of 17 students and staff last month.

Cruz, 19, chose to remain silent during a hearing in Broward County circuit court, where Judge Elizabeth Scherer on his behalf for 17 counts of first-degree murder and 17 counts of attempted murder for the shooting spree at Marjorie Stoneman Douglas High School.

Cruz, seen wearing an orange jail jumpsuit with his wrists and ankles shackled, sat alone and kept his head bowed throughout the hearing.

He sat in what is normally the jury box, guarded by a dozen police deputies.

Nine relatives of victims of the attack were in attendance, with one heard calling Cruz a “son of a b–tch” as he was led into the courtroom.

Judge Scherer set April 27 for a hearing for attorneys to agree to a trial date.

Cruz’s arraignment came only hours after students across the US conducted a 17-minute walkout in honour of the 17 victims.

Students in their thousands spilled out of classrooms, waving signs and chanting slogans in protest against gun violence prompted by the Florida massacre.

The protest coincided news of a teacher at a Northern California high school accidentally fired his gun inside a classroom, injuring three students.