OAKLAND, Calif. – A second-grade teacher in Northern California was placed on leave while a school and police investigate accounts by students that classmates engaged in oral sex and stripped off some of their clothes during class, officials said Friday.

The investigation was under way at Markham Elementary School in Oakland, where the principal notified parents of the situation in a letter 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 the 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 male teacher, whose name has not been released, told investigators he did not see any of the acts that authorities suspect occurred last week. The teacher is barred from campus at least until the investigation is completed.

The principal learned of the allegations Wednesday after a student 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," Principal Pam Booker wrote 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 students who engaged in oral sex, district officials said.

"I apologize for this and assure you that we are collaborating with counselors 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 in the letter.

Counselors were at the school Friday to speak with students.

District officials emphasized the students 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.

Some parents said they were outraged when they received the letter and saw the story on the news.

"It kind of scares me to know that the teachers aren't really watching them," said Ane Musuva, who has two children at Markham. "I don't want my kids growing up in this type of environment."

http://news.yahoo.com/s/ap/20110122/ap_on_re_us/us_classroom_sex