The 700 students at Bishop McNamara didn't know what to expect from First Lady Nancy Reagan's first visit to a Prince George's County high school, and the hallways were humming a week ago with last-minute preparations.

But Erik McDonald, an excited ninth grader from Forestville, was certain about one thing--her dress. "It will be a red dress; it's her favorite color," predicted McDonald, who was waiting for Reagan and Redskin player Mark Murphy to address the students about the danger of drug use.

Scurrying about the immaculate white-brick building giving gentle orders to staff members, principal Brother Walter Kramar still could not believe the first lady was on her way to Forestville, nine miles from the White House.

"It was an invitation which we didn't think she'd accept," Kramar said. "But it was right up Pennsylvania Avenue extended, and it fit right into her schedule."

When the 12:45 bell rang, the students poured out. They included 50 blue-tartan-skirted girls from La Reine High School who were visiting the all-boys school for the event. But there was also a horde of technicians and cameramen setting up in the gym where the assembly would be held. Scowling Secret Service agents combed the halls.

"This school doesn't even have security guards," said junior Pete Cevenini, 16, a clarinetist waiting nervously in the hallway with other band members.

"That's why it's so strange to see guys with guns, handcuffs and three-piece suits," Cevenini said, checking his watch to see if it was 1:15, the time for the 55 bandmembers to take their places.

"They said that nobody can move after 1:15," he said.

Reagan had accepted the invitation to be the keynote speaker for the biennial "Free Form Day," a daylong series of seminars and discussions on subjects of topical interest, nearly three weeks before that day. For security reasons, school officials were to keep the news from students until three days before the assembly.

But because the school had been closed by the blizzard, the visit wasn't announced until about two hours before Reagan was to arrive. As a result, the band didn't get much time to practice some of the special sheet music handed out that morning.

" 'Hail to the Redskins,' you know, all those patriotic songs," Cevenini said. "A little ordinary but if they like it . . . I mean, she must hear 'God Bless America' in every high school she goes to. We should play something original."

As it turned out, the band played its entire repertoire for the more than 700 students, teachers and guests while they waited for Reagan to show up. When she entered with Murphy around 1:45 to a rousing rendition of "Hail to The Redskins," the students jumped to their feet with cheers, whistles and applause. She was dressed, as it turned out, in a tweed. Murphy was in powder blue.

Student government president John Hines, who introduced Reagan, said later he thinks she is "bringing class to the White House" and added he was "disappointed that she didn't kiss me. I thought it would boost my image at La Reine."

Reagan reiterated the warning against drugs she has made at drug treatment centers and a few schools elsewhere in the county. Then she brought on Murphy, who told the audience that drugs and success do not mix.

The lightest moment came in the short question-and-answer period. A timid freshman asked Reagan where students could go for help with drug problems. Reagan said parents were the best people to first ask.

"Parent groups I'm very high on," said Reagan, and a knowing laughter slowly filled the gym.

"You know what I mean," Reagan recouped, blushing.

"Yeah, we know what you mean," said a wag wearing sunglasses in the last row--but not loud enough for the Secret Service men behind him to hear.

Later, she asked if the students had seen a recent "Little House on the Prairie" television episode that dealt with drug addiction.

"No," the students answered, with a groan at the mention of the homespun family series.

Some students did not feel they had much in common with her lifestyle either.

"She's been rich all her life. What does she know about drugs, about what is going on in the streets?" Sean Woods, 16, a junior, asked later in talking about the distance between the White House and his home in Oxon Hill.

" President Reagan policies hinder what she's talking about," said Andre Hodges, a junior from Seat Pleasant, noting a recent administration directive requiring parental notification of youngsters receiving birth control devices from federally funded clinics.

"Seems to me he's out to get teen-agers with those policies," said Hodges, a husky varsity football player. "They're going to mean more babies and that frustration will lead to more drugs, so how can I believe her?"

Before she left the stage, Reagan was presented with a dozen red roses, two maroon-and-gold McNamara T-shirts--"One for you and one for your husband," Kramar said--and a pin making her an official McNamara mother. The students went wild when she hugged and kissed junior class president Tommy Buckley, the T-shirt bearer.

"Now that I'm a school mother I can do that," Reagan said, just before she left in a phalanx of Secret Service men.

When it was all over, chairs put up, hot lights dismantled and instruments put away, Erik McDonald, who played the baritone horn, did not even notice that Reagan had not worn red. Instead, McDonald, like many students, was more impressed by her charm.

"I was expecting her to be just straight-faced but she appeared really casual," he said. "I wasn't ready for that" CAPTION: Pictures 1 and 2, Nancy Reagan with Brother Walter Kramar, principal of Bishop McNamara High School, where she warned students about drug use. [S]tudents applaud Reagan and Redskin player Mark Murphy. Photos by HARRY NALTCHAYAN--The Washington Post