Since the killings at Columbine High School seven years ago, schools have locked their doors, posted adults at entryways and drilled students in emergency procedures.

The result, say experts, has been an increase in safety, but there remains little that can be done to stop a distraught outsider intent on killing children.

William Pollack, a Harvard University professor who has done studies of school shootings, estimates that a child in this country has a one-in-a-million chance of being shot in an attack or suicide at a school.

"It is horrible, but it is minuscule," he said.

Fights and stabbings, in which one student targets another, remain the most common form of violence in schools. Students who kill indiscriminately are far more rare, occurring on average only eight or nine times a year across the country.

Even rarer is the type of shootings perpetrated by adults that occurred in Colorado and Pennsylvania over the past week.

But if parents and teachers overreact to the events of the past week by locking schools down -- requiring students to go through metal detectors and installing other security devices -- they are likely to traumatize children, Pollack said.

The best way to prevent school killings is for teachers and administrators to establish an atmosphere of trust so students report promptly signs of danger, whether troubled students or intruders, said Kenneth S. Trump, president of National School Safety and Security Services of Cleveland.

"There is no perfect security, but the first and best line of defense is a well-trained and highly alert teacher and student body," Trump said.

His organization tracks news reports of school-related violent deaths, from suicides to mass killings. Since Columbine, the group says, 219 people have died violently in or on their way to schools, or at school-sponsored events.

The annual totals range from a high of 49 in the 2003-2004 school year to a low of 16 the year before. Last year there were 27 killings and suicides.

While urban schools would seem to be more prone to school shootings, in fact, such violence rarely occurs there. The most infamous shootings have occurred in rural and suburban schools where parents, teachers and principals don't believe violence will ever happen.

"It has been our experience that there has been a heightened sense of preparedness in urban schools, but in suburban and rural schools there is a mind-set that 'It can't happen here,'" Trump said.

Monday's shootings in Pennsylvania reverberated throughout the region yesterday, prompting several Maryland school systems to re-examine their security procedures.

Carroll County public schools -- which currently leave the doors unlocked at front entrances -- might consider locking all school entrances, according to Larry Faries, coordinator of security for the system.

"You want schools to be a welcoming environment. But given the state of affairs, I don't know if we can do it," Faries said.

Police presence
In Anne Arundel County, Superintendent Kevin M. Maxwell visited several county schools yesterday to look at security measures and asked county police to increase the number of times they drive by schools during the day.

Maxwell also told principals to review their security procedures.