For parents, it is a nightmare scenario: Terrorists strike in the Washington area while their children are at school. The first thing to do, authorities say, is to resist the instinct to race to school to pick up your children.

Instead, school officials urge parents to immediately try to learn -- by radio, television, telephone, computer or other communications device -- what schools want them to do. In many scenarios, parents would be safest by staying where they are, while their children will be safest inside their brick schools.

"The idea that parents will not be able to come get their kids is difficult, but we want, we need, parents to understand this," said Paul Regnier, spokesman for Fairfax County public schools.

School systems across the Washington area are preparing for a terrorist attack, refining crisis plans used last year during the three-week sniper siege. School officials say, though, that there are limits to what they can do because nobody can predict what might happen.

"The most important information for parents to understand is that flexibility in responding to the unknown is the greatest piece of the readiness plan that we are now putting into place," said Brian Porter, spokesman for the Montgomery County school district, whose plans have been praised by federal officials.

At schools across the region, students are taking part in drills, sometimes evacuating the building and other times practicing what to do if the decision is made to stay indoors.

In some schools, for example, students are being told to quickly go into closets if a Code Red -- the highest state of threat on the federal government's advisory system -- is announced. Others are learning to sit next to walls, away from windows, and be quiet. Some schools, such as Chesterbrook Elementary in McLean, have asked students to bring in a change of clothes and some snack food.

Key school personnel are being trained to react to biological, chemical and other attack scenarios, and schools are being equipped with food, water and other equipment to help them ride out an emergency.

In Fairfax County, for example, teams from every school -- including the principal and the principal's secretary -- are undergoing training in which they must react to different attack scenarios. D.C. school officials have also trained school teams and staged a system-wide evacuation drill, said Yvonne Morse, principal at Davis Elementary School.

Every school district has some information on its Web site, and many schools and related organizations are sending home information on how to handle an emergency.

For example, McLean High School parents received a newsletter from the Parent Teacher Student Association that includes a message from Principal Donald J. Weinheimer Jr., who says explicitly that in the event of a chemical weapons attack in the community, he would likely be directed to bring all students and staff members indoors, shut down heating, ventilation and air-conditioning systems, and to close and secure all doors and windows.

But while districts are sometimes exchanging notes, they are not working together. They all have different plans, amounts of stored food -- something that is true at different schools in the same district -- and outlooks on what to expect.

Loudoun County officials, for example, say they are preparing for a chemical weapons attack because they view that as the most likely scenario, said district spokesman Wayde Buyard.

Porter said Montgomery County officials are basing their plans on the premise that school officials will not have the expertise to handle a nonconventional attack without the assistance of highly trained fire and other personnel, and will concentrate first on protecting children and dealing with their mental health and emotional needs while waiting for outside assistance.

Private schools, too, are taking steps to prepare in the event of an attack, to varying degrees. Some, such as Landon School in Bethesda, have detailed plans, heightened security and stockpiles of food.

There are some things parents can do now. They can make sure their child's emergency response cards at school are up-to-date and learn what identification they or a designee should carry to pick up a child. They can also learn about their school's individual plan -- and press school leaders if there is not one in place -- though realize that public school principals are not allowed to provide some details.

And school officials say that parents should tell their children to follow instructions. Buyard, for example, said some Loudoun parents have instructed their children to go home even if a school is locked, which he called unacceptable. "We won't let the children leave," he said, saying that in some cases, the doors will open for no one. "Some parents are going to be upset with that, but this is a matter of collective and individual safety."

In the event of an attack, there are a number of reasons parents should not try to get to school immediately. Schools, with their brick walls and thick doors, may be among the safest buildings. Roadblocks may be up, keeping parents out of doors unnecessarily. Children may be immediately evacuated, so parents could arrive to an empty building. And school officials may not open the door -- even to parents banging on it -- if there is a possibility that the parents have been contaminated by being outside.

Parents should also not assume that a school is open because the attack occurred miles away; schools across the area may go into "lockdown" until danger of a further attack seems to have passed, school officials said.

"We hope that parents will be sheltering in safe places at home or at work, because the most prudent response for everyone is to stay inside in secure spaces," said a letter to parents from Patricia A. Weitzel-O'Neill, superintendent of the 110 schools in the Archdiocese of Washington, which encompasses the District and Southern Maryland.

The issue of how children will communicate with their parents is a tricky one. School officials say they realize it is impossible to ban cell phones -- and many children younger than 8 are being given them by their parents -- but that the system could get overloaded. Schools are now trying to develop multiple methods of communication, which include announcements on radio, television, Web sites and special e-mail announcements. But officials in school systems across the region say that parents should not expect to be able to speak with their children immediately.

In some districts, some school personnel will have BlackBerries -- Paul Regnier, the spokesman for Fairfax County public schools, now carries one on his belt -- hand-held devices that can carry text information independently of phone networks, but nobody is suggesting that children carry such equipment.

Another key for parents, Morse said, is to keep calm. "The less panicky the adults are, the better the kids will be. In fact, I think they can handle it better than we can."

Federal officials are also focusing on school preparedness, and school officials and parents can look at www.ed.gov/emergencyplan/ for more information on the Department of Education Web site.

Schools across the region are refining crisis plans originally drawn up during the area's sniper scare. Here, Tasker Middle School Vice Principal John Kenty, center, questions a Secret Service instructor during training for school personnel on how to deal with sniper situations.Davis Elementary Principal Yvonne Morse said D.C. schools have held an evacuation drill.