It was not the typical school board letter. Manassas City School Board members wrote to parents this week that they were "shocked and offended" to learn that there had been sexual activity among a group of students inside the Osbourn High School auditorium.

The students, board members wrote, "gathered freely and voluntarily" in the closed and darkened auditorium after school at 5:30 p.m. Sept. 28. Board members said they "had not considered the possibility that students would engage in sexual acts while at school."

The School Board vowed that the school system's code of conduct would be revised and security would be tightened. The Manassas Police Department investigated whether a sexual assault had occurred.

In a statement released yesterday, police said that the case was being categorized as "inactive" but that it would be reopened if additional evidence emerged. "The investigation has determined that the sexual activity among the students did not reach the threshold of prosecution," Sgt. Tim Neumann, a police spokesman, said in the statement.

But the rumor mill is not inactive. For the past three weeks, parents said, they have asked one another: What was the sexual activity? And how could it have happened within the insular walls of a school building?

According to sources familiar with the incident, the students -- two girls and six boys -- each received suspensions. Both girls and three of the boys engaged in sexual intercourse or had oral sex while the other three boys watched, said the sources, who spoke on the condition of anonymity because of the sensitivity of the case.

The lights of Osbourn's auditorium were dimmed low, but the students were caught when a janitor walked in on them, the sources said. School officials used video surveillance cameras outside the auditorium to determine which students had participated, the sources said.

For parents, details of the incident emerged in a gush of half-truths and vague official statements. The incident challenged their belief that something so salacious could never happen in their suburban school system.

"This is a small, tight-knit community. There are some school systems that wouldn't blink an eye at something like this," said Trisha Spitler, whose daughter is a freshman at Osbourn, the city's lone high school. "It's just very protected, you know. Everybody is always on top of things. We're very blessed and fortunate to have good people."

Some parents felt that the school system or police should have released more information so they could assess whether their children were safe, said Tim Demeria, whose 14-year-old daughter is a freshman.

"I wished we got information. Was it criminal? Was it consensual? I know police had to do their investigation, but I would have liked to know what happened quicker," Demeria said.

The school's principal, Perry B. Pope, said he understands parents' concerns that more information was not released. "Since this was a student discipline issue, we could not discuss details, and parents were frustrated. And we were frustrated because we could not give it to them," he said.

School Board members said they viewed the incident harshly, noting the contrast between their generation and the current one.

"I don't think it crossed anyone's mind that you would have high school students sneaking off into the auditorium for sexual activity. That's where we have assemblies and bring parents in," said School Board member Patrick D. Linehan.

"We wouldn't have seen this 30 years ago when I was in high school. Then again, maybe I am naive and I wasn't one of those guys."

School Board member Patrick D. Linehan found the incident astonishing.