A male teacher at Oakland's Markham Elementary School has been suspended with pay, Oakland Unified School District spokesman Troy Flint told KTVU. He did not identify the teacher.
"I think everyone is taken aback over this shocking incident," Flint told KCBS, Channel 5 in San Francisco. "Of course, it is hard to understand how that could have occurred."
"According to the allegation, the first incident involved several students who undressed, at least partially, while acting disruptively in the classroom," the letter read, according to KRON. "The second incident concerned two students who engaged in sexually explicit behavior, also in the classroom."
Local media reported that in the second incident, a boy and a girl -- both second-graders -- performed oral sex on each other. Both incidents occurred in the same classroom, and the teacher is believed to have been present at the time, Flint said. He said the teacher is not accused of taking part in or orchestrating the incidents.
"He's saying he has no knowledge of the reports and we have to square these different stories and see how we can reconcile them," Flint told KGO, the ABC station in San Francisco.
Some parents blamed the teacher for lack of control over the classroom.
"You have a beautiful principal here who holds her school down. I just can just say it was the ignorance of that teacher," parent Lataya Young told KGO.
Others said the children must have been acting out things they see at home.
"I still don't know how that could happen," parent Alan Saunders told KTVU. "They're imitating what they're seeing. They didn't learn it at school."