Last spring, a sixth-grade Montgomery County girl was thrown down in the back of her school bus by several older boys who, the girl said, grabbed her breasts and buttocks and feigned sex acts.

In December, a 6-year-old Frederick County girl was allegedly fondled by a middle-schooler while riding a bus to her gifted student program. Her mother said she didn't learn of the incident until May, when the driver told her.

Two months ago, an 11-year-old girl was allegedly attacked by two girls and three boys during a bus ride home from her elementary school, south of Richmond. The group, the girl said, held her down, groped her and penetrated her with an object.

Every day, 440,000 school buses ferry 18 million children to and from schools and activities across the United States. Accidents, seat belts and safe crossings generally are the matters parents worry about. But experts say sexual assaults on school buses, one of the fastest-growing forms of school violence, seldom register as a safety concern.

Although many school systems don't identify bus assaults independently of all school violence, administrators, teachers and bus drivers say the nature and frequency of the attacks are increasing, and at younger ages. The incident involving the Germantown girl was one of four alleged sexual assaults on Montgomery County's school buses this school year; the alleged attackers in Virginia were as young as 8.

A 2001 report commissioned by the American Association of University Women found that eight of every 10 students in grades 8 through 11 report having been sexually harassed at school, most often by peers. One-third of students surveyed said they were first harassed in grade school.

"I've never experienced the problems and the degenerate actions of kids as I have this past year," said bus driver Bob Baxley of Hagerstown, Md., who has been driving school buses for 12 years. He was driving during the alleged attack in Frederick. On the same bus, Baxley saw middle school boys describe sex acts to first-graders and one boy try to shove a condom into another child's mouth.

"Sexual harassment is a much more serious issue in public schools than most people have been willing to admit," said Robert Shoop, a professor at Kansas State University. "And it's much more likely to occur in unsupervised venues -- like buses or bathrooms."

Buses are "more dangerous, in that society has become more sexualized and less civil," Shoop added. "Now many more kids are saying, 'I don't want to ride the bus.' They're scared."

Yet, he said, only about 5 to 10 percent of students report being victimized. "It takes a lot of courage on the kid's part and money on the parent's part" to press forward with charges, Shoop said.

Schools, safety advocates say, need to improve school bus supervision, teach students about appropriate behavior and encourage students to report incidents that cross the line. When incidents do occur, quick and decisive action by schools and police is vital, Shoop said. "Kids have to see there are consequences," he said. Otherwise, "they don't see it as serious."

Sometimes neither participants nor victims know when bad behavior becomes criminal.

In March last year, Jessica Killian found her daughter Ashley, then 11, crying in her bedroom in the family's Germantown home. She told her mom her stomach hurt. She couldn't bring herself to tell her mother that on the bus ride home from Roberto Clemente Middle School that afternoon, six older boys allegedly held her down on the bus floor, groped and lay on top of her. It wasn't until the next day, when she got on the bus and the boys threatened to "finish what they started," her mother said, that Ashley told an assistant principal what happened. The school called her mother, and her mother called police.

Two students were transferred, two others suspended. A police report was completed, and the state's attorney's office recommended charges. But the case file was misplaced by police. By the time it resurfaced, the statute of limitations governing the crimes had expired.

"We recognize we made a mistake" in the case, said Lt. Eric Burnett, a Montgomery County police spokesman, and the department is working to ensure it doesn't recur.

Since the beginning of the school year, there have been 26 alleged incidents of sexual harassment on school buses in Montgomery, including one two weeks ago involving five students at White Oak Middle School and one in February at Clemente, system spokesman Brian Edwards said. Only four were serious enough to involve police, but the incidents have spurred school officials to take action.

In May, school administrators formed a work group to help improve communication among bus drivers, schools and the central office. In the fall, school officials will launch a campaign to educate students about proper bus behavior. They are also considering whether to add cameras on buses; of the 1,200 buses in the system's fleet, only 20 are equipped with cameras.

Clemente Principal Rosalva Rosas said she's found that middle-schoolers, on the cusp between child- and adulthood, "don't always know how to act appropriately."

That seemed the case in December in Frederick, when, in the last few days before Christmas vacation, the 6-year-old girl, bundled in her lavender parka, climbed aboard the middle school bus that shuttled her to a gifted program at Urbana Elementary School. For two 40-minute trips each day, the child was one of two or three first-graders on a bus for middle-schoolers.

That winter morning, the girl was sitting in a front seat with a classmate when one of the middle-school boys sat nearby and began "tickling her" in a way that "could have been construed" as sexual harassment, said Baxley, her bus driver. The girl later said the tickling included her "private parts." In early May, Baxley mentioned the incident to the child's mother, who hadn't known about it.

"He called it 'fondling,' " the girl's mother said. "I was crying and shaking." She said the driver told her that he'd told school officials, who say they have no record of that. Last week, Frederick County police detective Willie Ollie said no charges would be brought because statements from the girl and Baxley did not support a sexual assault charge. But the girl's family disputes that; they have yet to decide their course of action, said the family's attorney, Eugene Souder.

Schools often handle discipline issues internally, making it hard to judge the scope of the assault problem, said Beverly Glenn, executive director of the Hamilton Fish Institute in Washington, which studies school and community violence. "Under No Child Left Behind, you don't want your school to be characterized as an 'unsafe' school," she said, because "you lose students, you lose money."

Other districts, however, believe swift and decisive action is the way to prevent such incidents from becoming regular occurrences.

It was a Friday afternoon and the bus was full when an 11-year-old girl boarded it for the trip home from Lakeview Elementary School near Richmond. Before the first stop, three boys and two girls ages 8 to 13 allegedly grabbed the child, held her down in the back of the bus and assaulted her with an object.

The driver saw the incident, notified supervisors and the school, drove the child to her stop and told her mother. Police and the school coordinated investigations: Colonial Heights School Superintendent Joseph Cox told local media that he wanted "the maximum discipline available" for the children, who were placed on house arrest, with electronic monitoring.

Within a week, police announced they were seeking charges ranging in severity from abduction to aggravated sexual battery.

Immediately after the incident, adult patrols were put on all Colonial Heights school buses. Two of the children involved have been expelled, and the School Board plans action on the remaining three.

"I went through this school system, and I couldn't even imagine that something like that would've taken place," Cox said. Now that it has, "we're going to go back and do better."

Staff writers Joshua Partlow, Ian Shapira and Nick Anderson contributed to this report.

Ashley Killian, 12, shown with mother Jessica and brother Alejandro, 6, was allegedly groped on a bus.