Schools in Montgomery and Prince George's counties took on the feel of fortresses yesterday as police helicopters whirred overhead and officers joined with firefighters to set up a veritable blue wall of force to protect against a serial sniper who shot a 13-year-old student Monday.

The show of force was designed to reassure frightened parents and students returning to school after a sniper had killed six adults and wounded another then targeted and critically injured a child.

But many parents said the armed presence heightened their sense of vulnerability as they walked their children to school, hand in hand, or stood with them at bus stops as they anxiously looked over their shoulders, or waited in unusually long carpool lines. And false reports of danger popping up throughout the day kept people on edge.

Fear and uncertainty were hardly limited to Montgomery and Prince George's counties.

Across the region -- including in the District, in Fairfax and Prince William counties in Virginia and in Southern Maryland -- after-school activities and field trips in most schools were again canceled, and students were kept indoors.

Parent Susan Seabrook said she was pleased that Sligo Middle School in Montgomery, not far from the sites of several of last week's shootings, rerouted traffic so parents could drop off children only at the front door instead of at the usual location behind the school, near a densely wooded area. Still, she said she was uneasy.

"I don't know what else they could do," she said, gesturing to the Montgomery patrol car idling nearby and the school in full Code Blue lockdown mode. "But it's just the unpredictability. Everything's become more harrowing."

Students returned to Benjamin Tasker Middle School in Bowie -- the site of Monday's shooting -- under tight security, a large media presence and intense scrutiny by parents. Four police cars were parked outside the front entrance, and security guards allowed into the parking lot only parents, students and public officials.

Many students who ordinarily go to school on their own or on a school bus walked onto campus clutching their parents' hands, and parents said they would escort their children until the sniper is caught. "I will go to work late -- whatever it takes," said Marlene Biggs as she accompanied her son, Patrick, 13.

Attendance was about 70 percent, down sharply from the average of 95 percent, and many other parents said they considered keeping their children home but wanted to show that they were not afraid.

There were jitters at other Prince George's schools as well. At High Point High School in Beltsville, for example, afternoon dismissal was delayed almost an hour after police were called to check out a report -- determined later to be false -- that a man with a gun was nearby.

In Fairfax, some parents said there was a "false sense of security" in the area because no shootings had occurred there. But others said they were full of fear. Jan Huddleston's daughter usually walks to Fairfax High School each day, but for the first time, Huddleston told her daughter to ride with friends yesterday.

"If they don't pick you up," Huddleston told her, "I'll drive you."

At Calvert County's Patuxent High School in Lusby, more than 50 miles from any of the shootings, Principal Robert F. Dredger said students seemed absorbed in their lessons. But even there, a sense of peace was elusive.

"When you're dealing with a sniper, you know in the back of your mind that you're vulnerable," Dredger said.

In Montgomery County, where schools had long been scheduled to be closed today, authorities set up a new Web site for parents, employees and others to provide a single point of reference for news about the emergency,

At Albert Einstein High School, Georgia Ploussion, 14, said she feels scared all the time. As a police officer wearing combat boots and carrying a machine gun peered into each car in the parking lot, his radio crackling with false reports of gunshots, the heavy police presence just made her more jumpy.

Classmate Nemtsa Falu was afraid to walk home. And Lena Ouk has been having trouble sleeping. "There's a lot of danger out there," Lena said.

At school, their freshman honors English class has been moved from a portable classroom to the cafeteria, where the class watched a film version of John Steinbeck's "Of Mice and Men."

Einstein Principal Janice F. Mills said she made the move because the portables are "too isolated, and the woods are right there."

Georgia -- and other students -- were not permitted to use the restrooms unless escorted by an adult. And at home, her parents will not allow her to go out with friends to the movies or shopping mall "until it's all over."

At Highland Elementary School near Wheaton, Joe Scott was one of thousands of parents who, officials say, brought their children to school or helped get children there safely, because the fifth-grade safety patrol had been temporarily disbanded.

As he directed traffic and helped firefighters who had set up rows of orange and yellow cones to restrict access to the parking lot, Scott said he worried not for his children's physical safety, but for their emotional well-being.

"My middle school son has become obsessed with the news to the point where it's harmful," he said. "He won't sleep in his room anymore. He sleeps upstairs on the couch."

Gina Foster, late for work as an elementary school teacher, rushed past the police line to talk to her son's counselor. "In my heart of hearts, I don't think anything can happen to him," she said. "But how can you protect their hearts from fear? That's what breaks my heart."

Staff writers Christina A. Samuels, Liz Seymour, Theola Labbe, Avis Thomas-Lester and Justin Blum contributed to this report.