Cedric Anderson, 53, of Riverside, entered a classroom at North Park Elementary School and opened fire on his wife, Karen Elaine Smith, about 10.30am before taking his own life, according to San Bernardino Police Chief Jarrod Burguan.

Two students standing behind Smith were also shot, police said. The boy, identified as Jonathan Martinez, was airlifted to Loma Linda Medical Centre, where he was later pronounced dead. A nine-year-old student who was wounded remains hospitalised in stable condition, Burguan said.

The shooting occurred in a special education classroom for students with intellectual disabilities, said San Bernardino Unified School District spokeswoman Maria Garcia. There were 15 students from the first through fourth grades in the room, and two adult aides, Burguan said.

The chief said the couple had only been married for a few months, and he described them as "estranged." Burguan said Anderson was armed with a high-calibre revolver.

The gunfire was reported at 10.27am in a classroom at North Park Elementary School. San Bernardino Police Captain Ron Maass said the shooter checked in with school officials before visiting the teacher's classroom, but no one saw the handgun he was carrying until he opened fire.

It is unclear how many shots were fired, Maass said.

"The children, we do not believe were targeted," he said.

Jaidyn Stanley, nine, said he was in a different classroom when the shooting happened.

"I was in my class and my teacher was teaching us a lesson, and then I heard three gunshots. My teacher told us to get on the ground. Then we started hearing sirens," the third-grader said.

Jaidyn said after staying low to the ground for about 30 seconds, his teacher told the class to get up, run and follow her out of an emergency exit that connects directly to the outdoors. He and his classmates left their backpacks behind.

"There was a lot of people in my class crying and they were scared. They thought the shooter was going to come in the classroom," Jaidyn said.

Jaidyn said once he and his classmates were outside on a soccer field, they were planning to walk to Cajon High School, but he spotted his mother and she scooped him up and took him home.

North Park Elementary is a magnet school for students in pre-kindergarten through sixth grade who are interested in environmental issues, according to Garcia. Students take field trips to outdoor areas and do school work "with an emphasis on science activities that nurture creative expression.

Armed security officers are not assigned to district elementary schools, according to Garcia, but security remains "very, very tight" on campus.

"Once the school bell rings, the only way in to the campus is through the front office," she said.

All district schools maintain ledgers for parents, volunteers and district staff, she said, and everyone is required to show a photo ID and sign in at the front office. The gunman, she said, "followed the check-in protocol."

Immediately after the shooting, the San Bernardino County Fire Department set up a triage area.

School officials said the shooting was "isolated to the campus." In an email to staff, the school district said: "This is believed to be a case of domestic violence."

Students were evacuated to California State University, San Bernardino's physical education building, where they could access bathrooms and water, said university spokesman Joe Gutierrez. San Bernardino Police also tweeted images of children being given glowsticks and other toys throughout the day, and some parents said their children were allowed to watch movies while they waited to be released.

Parents were directed to Cajon High School, where officials verified their identities before sending them to the university to pick up their children, Gutierrez said.

North Park Elementary has more than 500 students between kindergarten and sixth grade, mostly from low-income Latino families.

Students were huddled on a field at a corner of the school's campus on Northpark Boulevard and H Street, accompanied by teachers and guarded by law enforcement officers carrying long guns.

Anxious parents such as David Zamudio gathered nearby, but barriers blocked them from reaching their children. Some parents said there was confusion over where to collect their children as information circulated that they should be picked up at either Cajon High School or Cal State San Bernardino.

Zamudio, the father of a six-year-old in second grade at North Park, said he lives nearby and heard helicopters overhead. He rushed to the school when his sister called saying there had been a school shooting.

"I came because they said it was safer, more isolated. But I guess it's not that way," said Zamudio, who recently moved to the area from nearby Highland.

In a statement on Twitter, US Secretary of Education Betsy DeVos said: "My heart and prayers go out to the victims of today's horrible act in #SanBernardino & to the whole North Park Elem. School community."

According to the state's new school rating system, North Park earns high marks for suspending less than 1 percent of its student body. The school was deemed yellow - average on the state's color-coded grading scheme - for academics. In both math and English, students scored below the bar for proficiency, but in math, their scores grew significantly over the course of one year.

There were 385 mass shootings in the United States in 2016, according to the Gun Violence Archive, which compiles such incidents. The organisation defines a mass shooting as an single shooting incident in which four people are killed or injured, including the gunman.

There have been 219 School Shootings in the United States since 2013, according to Everytown for Gun Safety, a group that tracks gun violence in the US. The group reports that, on an average day in the US, seven children and teenagers - age 19 or under - are killed with guns.

The shooting comes as San Bernardino has seen a major increase in violence.

There were 62 slayings in San Bernardino in 2016 - a 41 per cent increase from the year before. It was the deadliest year in the city since 1995.

The violence is an open wound on a city trying to recover from a prolonged bankruptcy and the 2015 terror attack.

The school is about 16 kilometres from the site of a terror attack in 2015.

Reuters, Los Angeles Times