OAKLAND, Calif. - The morning after receiving a letter about "sexually explicit behavior" that may have occurred in a second-grade classroom last week, parents arrived at Markham Elementary School wondering what had taken place and whether the rumors were true.

Maria and Susana Garcia, sisters whose children are in kindergarten and first grade at Markham, said the reports made them feel uneasy and worried.

"You leave your child at school because you trust that they're going to be safe," Susana Garcia said Friday.

Oakland school district officials say the principal received a report Wednesday that two second-graders engaged in oral sex during class while the teacher was present, and that -- also that week -- some students from the same classroom were clowning around, partially undressed.

The teacher is on paid leave while the investigation is pending. He has said he was unaware of either incident, said Troy Flint, a spokesman for the Oakland Unified School District.

"If these reports turn out to be true, obviously there was an unacceptable lapse of adult supervision," Flint said.

Both incidents reportedly happened last week, but Flint said the school administration only learned about them Wednesday after a child came forward. Flint said preliminary interviews conducted by the school principal led officials to believe there was truth to the story, although he acknowledged that some of the details were vague.

He said the investigation is continuing with school district police officers and the city's special victims unit. Counselors have been brought to the school, he said, and the administration was holding meetings with parents.

There are 20 children in the second-grade class, he said.

As Flint spoke, a man stormed out of the school building. "Incompetent!" he yelled. And then, pointing to the news trucks that crowded the narrow side street off 73rd Avenue, "Sensationalized!"

Dr. Stuart Lustig, a child and adolescent psychiatrist at the University of California, San Francisco, said sexual play among children -- though often worrisome to adults -- is not uncommon, even at a very young age.

"They do experiment with each other's bodies," he said. "It's part of normal development."

If the activities are coercive or repetitive, however, it raises concerns that a child has been abused or exposed to sexual activity or material, Lustig said. He also said oral sex is not as common, "and probably more concerning."

Gerardo Marin, a Markham parent, said he transferred his child to Markham from another Oakland school because of its good reputation. Its reading and math test scores have risen and are now just points away from the state goal. Marin said he has lived in the neighborhood for years, and that this is the first time he's heard of a major incident being reported at Markham.

Marin said the allegations, whether or not they're true, made him realize how little parents know about what happens inside the classroom.

Still, he said, "Maybe it's a rumor. I don't believe it, especially in the second grade."