A teacher at a California primary school has been suspended amid shocking claims that children engaged in oral sex and took off their clothes in class.

The principal of Markham Elementary School in Oakland notified parents of the situation in a letter on Thursday.

"We believe if the reports are true, there was a serious lapse of judgment or lack of supervision in the classroom," said Troy Flint, a spokesman for Oakland Unified School District. "We're investigating how could this have happened. It seems unthinkable to us, just the same way it does to the public."

The second-grade male teacher, whose name was not released, told investigators he did not see any of the acts that authorities suspect occurred last week. He has been barred from campus at least until the investigation is completed.

Principal Pam Booker learned of the allegations on Wednesday after a pupil gave an account to a teacher's assistant, Flint said.

"Upon hearing these reports, we immediately launched an investigation which, to date, suggests that the reports have merit," Booker said in the letter to parents. "We have interviewed all the student participants who were implicated, as well as their teacher, and we continue to investigate the matter aggressively."

One incident involved several students who partially undressed and acted disruptively during class, while the other involved pupils who engaged in oral sex, district officials said.

"I apologise for this and assure you that we are collaborating with counsellors and parents to provide support to those involved, address any concerns and take whatever actions are necessary to ensure that a similar act does not occur again," Booker told parents.

Counsellors were at the school on Friday to speak to children, who, the education authority stressed, were not accused of any wrongdoing.

"It's an incident of kids expressing their natural curiosity that went too far because an adult didn't step in," Flint said.