The five police officers edged forward, knees bent and guns raised, past the "Reading" bulletin board and down the hallway at Rockville's Richard Montgomery High School. They were creeping in a clump--three in front, one in the middle, the fifth walking backward, facing the rear--when the middle officer fell.

A shout of "Officer down!" ricocheted off the green tiled walls, and the downed cop was dragged away by his partner.

The guns were fake, the incident a re-creation, but the lessons Montgomery County police officers learned at the training session yesterday were dead-on serious: Schoolhouse walls make good ricochet chambers not just for shouts but for bullets. Police radios sometimes don't work in the middle of a large school. And doors don't always open the way one would expect.

Yesterday's session at Richard Montgomery and four other Montgomery County high schools was the first step in a new county effort to teach patrol officers the specific issues and challenges of responding to emergencies at big buildings, workplaces and, most of all, schools.

The 750 officers participating in what's known as "Critical Incident Training for First Responders" are the ones most likely to show up immediately after a 911 call, not the SWAT team officers trained specifically to deal with hostage situations.

With fatal school shootings in the last two years at such places as Paducah, Ky., Jonesboro, Ark., and Springfield, Ore., Montgomery officials had been rethinking school security since before last spring. But April's mass shooting at Columbine High School in Littleton, Colo., and the confusion and bloodshed that occurred even after police were on the scene, prompted Montgomery officers to design this training.

"In the past, the strategy was to isolate, contain and set up a perimeter," then wait 20 minutes or so for the SWAT team to come in, said Capt. Drew Tracy, one of the officers in charge of the Montgomery County effort. The old tactic works when an incident is over fast. But as Littleton showed, that doesn't always happen.

Now it's clear, Tracy said, that "you can't sit back while further lives are taken." The first responders, he said, may have to use superior numbers and firepower to end the crisis themselves.

Before they began the tactical exercises, the officers studied an 11-year history of school violence--in particular, the types of children, their weapons and their methods. Then they focused on details of the buildings: how the doors open, how to maneuver the stairways, the way a bullet might bounce inside, how the alarms work, how a school's thick, metal-framed walls can make it difficult to broadcast radio signals.

In addition, the police have compiled thorough blueprints of all the county's schools--to avoid what happened at Columbine, Tracy said, where the principal had to frantically scribble a map on a piece of scrap paper for police while the killers were inside. And officers are being urged to go to all the high schools and middle schools on their own beats, to familiarize themselves with the buildings.

Tracy emphasized that the police force is not trying to turn out a team of commandos: "We are not teaching our officers to blindly run into schools with weapons drawn."

CAPTION: A team of Montgomery County police officers moves into position during a mock shooting at Richard Montgomery High School in Rockville.