A six-year-old boy has been detained after allegedly shooting a school teacher in a first grade classroom in the US.

Police say the child had a handgun and allegedly fired a shot at the female teacher during an “altercation” at Richneck Elementary School.

The shot forced the school of about 550 students in Newport News, Virginia, into lockdown.

But authorities were quick to assure the shocked community it was “not a situation where someone was going around the school shooting”.

Although it was an isolated incident, Mr Drew explained the awful occurance was “not an accidental shooting”.

The injury to the teacher in her 30s was considered life-threatening, although she had shown some improvement in the hospital, Mr Drew told reporters.

There was no further explanation for how a six-year-old acquired the handgun.

“I want to know where that firearm came from,” said the police chief.

Schools superintendent George Parker said all school campuses were equipped for random metal detector searches, but they were not deployed at Richneck Elementary on Friday.

“Today our students got a lesson in gun violence and what guns can do to disrupt, not only an educational environment, but also a family, a community,” Mr Parker said.

Richneck Elementary School, with about 550 students, has metal detectors. Photo: Getty

Local officials vowed to find answers and prevent such an event from happening again.

No students were injured but they had been moved to a gymnasium for their safety and counsellors had been made available, Mr Drew said.

“They are safe. They are interacting with officers and having some fun, slapping some high fives and teasing each other.”

Students were reunited with their parents under a school escort.

Recently elected Newport News Mayor Phillip Jones promised a full investigation involving the city council, school board and others.

“We are going to ensure that we have the right steps and policies so that this doesn’t happen again,” said Mayor Jones.

A mother keeps her son close as they leave the school. Photo: AAP

Mr Parker lamented educators’ inability to keep guns out of school, saying he was “shocked” and “disheartened”.

“We need to keep guns out of the hands of our young people,” Mr Parker said.

“I cannot control access to weapons. My teachers cannot control access to weapons.”