The 10 suspects file in to face a burly plain-clothes policeman from behind the soundproof glass booth in the line-up room.

"You got to be more serious," the officer warns, speaking to the 10 through microphone. "You may go to jail for 10 years."

The witness to the crime stands beside the officer. The suspects behind the glass cannot see her, but she eyes each and every one of them up and down, from head to toe, slowly and deliberately.

"Keep your hands behind your back. Please look straight forward." the officer says through the microphone.

The scene is repeated daily at District police headquarters on Indiana Avenue. The suspects are almost always between 15 and 35 years old, and are usually wanted for minor thefts, like purse snatchings, according to Officer George S. Adams.

The difference last week was that the "suspects" were much younger than those usually in line-ups.They were elementary and high school students taking part in a demonstration of how the line-up operates, and the "witness" was their teacher.

The demonstration was just part of a tour of police headquarters that took the students to the police communications division, where calls are received and cars dispatched, and to the firearms identification branch, where more than 1,000 police revolvers, rifles and pistols are kept.

The station house tours - the main part of the department's observance of National Police Week - were primarily a public relation campaign aimed at boosting the image of police among young people.

And if "oohs" and "ahhs" are any indication, the tours were for the most part a success. The children all giggled on cue, made faces at their teacher and friends from behind the line-up glass, and were genuinely interested in finding their school on the large map of the city in the dispatch room.

"Why is the room so big?" someone asked. "What are the cameras for?" asked another.

But all novelties of the station house could not compete with the treat that got the loudest hurray - the sodas and cookies at the end of the tour, courtesy of the American Legion.

"It's a P.R. thing really," said Hy Wayne, the Legion's youth chairman. "A lot of these kids never see a policeman, unless it's out on the street."

Sgt. William Mann agreed. "They see the other side of it. On the street, they only see an arrest."

Fourteen school groups from around the city took advantage of the station house tours last week, including one Greek Orthodox school, and Amy Carter's fifth grade class from Stevens Elementary. Officer Adams said the department had to keep the Amy Carter visit "low profile."

The tours began when Adams met the students in the line-up room with a hearty "Good morning, boys and girls."

He then took the students through the scenario of a crime, usually a purse snatching - "the most common crime in Washington," he told them.

"Then let's say a pocketbook snatching has occured," Adams said. "Now every criminal has a modus operandi. Mo-dus op-er-an-di," he says again, pronouncing every syllable. "That's a strange kind of a word. What language is that?"

"Latin!" someone shouts from the back.

"Right," Adams replied. "You have a certain way you dress and comb your hair in the morning. That's your modus operandi."

Adams then told the children about the line-up-routine, and describes how the witness can see the suspect, but not the other way round.

When the children didn't believe him - and what inquisitive child would? - well, that's when Adams let them see for themselves. He asked for 10 volunteers to be suspects in a line-up, and the entire group of 30 or so would scramble towards the stage, each trying to shout "Me." louder than the next.

Adams usually had to remind them that the line-up is serious business, and he'd order number seven to stop making those funny faces, or ask number three to "look more mean." Before it was over, the entire group of suspects usually broke down with a case of uncontrolled giggles.

The tour - just over an hour long - ended back in the line-up room, where Adams announced the surprise - the cookies and sodas for everyone upstairs in the community relations office.

"They don't come in contact with the police except in the role of "big guy," said Nikki Clark, a teacher at the Seven -Day Dupont Park school. "This help them have more respect."

The students agreed.

"Most people say the police are mean," observed Noble Campbell, one of the "suspects" in the mock line-up. "But they're only doing their job."

Another student, Karen Jones, agreed. "I think all schools should come here," she said."It might stop people from committing crimes."