OAKLAND -- A second-grade teacher has been placed on administrative leave amid an investigation into a report that two of his students engaged in oral sex during class and that, on a separate occasion, some children took off some of their clothing and were clowning around.

The incidents happened last week at Markham Elementary School, but they only came to the principal's attention Wednesday, after one of the students told a staff member what had happened, Oakland school district spokesman Troy Flint said.

The teacher was present at the time, but says he was unaware of any such incident, Flint said.

The district conducted a number of interviews Thursday to verify the child's report. While some of the details remain vague -- such as when the two incidents occurred, and if they happened at the same time, Flint said. "We believe the substance of the story is true," he said.

Counselors have been brought to the school, and a letter went home to families Thursday, Flint said.

"Obviously it's a horrible situation for the kids, and it's a huge embarrassment for the school and for the district," Flint said.

"We are striving to create schools as safe havens where students can feel secure and trust in adult guidance," he added. "Obviously, that trust was violated in this incident, which was unacceptable. We're going to get to the bottom of this and make sure there's no recurrence. "