Take one auditorium, add 700 boys, 100 girls, one first lady and one Washington Redskin. Mix in "The Star-Spangled Banner," "God Bless America," motherly kisses and professional football talk and what you'll get is an apple pie, All-America 1983 high school assembly.

Nancy Reagan found that out yesterday at Bishop McNamara High School in Forestville, Md. So did Redskins safety Mark Murphy. To say Nancy Reagan, as keynote speaker, was "first" may have been technically true but there didn't seem to be any doubt that Murphy upstaged her--to her and everybody else's delight.

"I'm not used to this," Murphy demurred modestly after his introduction by Mrs. Reagan drew a thunderous standing ovation. "I haven't been a world champion too long."

Though the mood was exuberant, the message brought by the two visitors was serious. Murphy and Mrs. Reagan costarred at an assembly focusing on drug and alcohol abuse, an early kickoff to the school's "Free Form Day Agenda" being held today. The topic: "Justice and The Nuclear Issue."

"Today I brought along a player from some team," said Mrs. Reagan, pausing for effect, then added tantalizingly, "called the Redskins, who will give you his thoughts on the dangers of drugs."

Murphy's thoughts on the subject to the Catholic all-male student body, with 100 invited guests from Catholic all-girl Lorraine High School of Suitland, Md., coincided with Mrs. Reagan's. The presence of both coincided with the Ash Wednesday visit nine miles away at the White House of a congressional delegation to discuss with President Reagan the resubmission to Congress of the tuition tax credit, which the administration supports.

"In order to be successful," said Murphy, "you can't use drugs . . . I would like to tell you a little about myself. I have never tried marijuana, never tried cocaine, I've never used any drugs. I don't say that to try to impress upon you that I'm a goody-goody and I wouldn't touch it. I've been around drugs, obviously, as a professional athlete but even more so in college in the early '70s when marijuana was so prevalent.

"It's hard. It's a question I've asked myself: 'Why didn't I use drugs?' A lot of it was I was always so busy that I just didn't have time for it. What I've noticed is that peer pressure forces people into using drugs . . . but I rebelled against that. I didn't want to be told by other people to use drugs . . . A lot of you kids are probably in that situation right now and it's probably very awkward to say no. But looking back I feel so much better about myself . . . You have to have a little bit of foresight, look ahead 10 or 15 years to what's going to happen . . . the thing you have to ask yourself is, 'Is it worth it to take the risk?' "

Questions were drafted in advance and assigned to McNamara students by Brother Walter Kramar, principal. They ranged from whether there is a greater percentage of drug problems in professional sports than outside it, to where a youthful drug user can get help.

Hugh Turley, 14, a freshman, said later he would have asked the same question about where to get help even if it hadn't been prepared in advance. A school official identified Turley among those students who suggested that Mrs. Reagan be invited to McNamara to discuss drug abuse.

Murphy--whose wife, Laurie, eight months pregnant, was introduced from the audience--was rewarded with a McNamara T-shirt (extra large) and cap. Everybody cheered when he donned the cap. And Kramar made Mrs. Reagan an honorary McNamara mother.

When two students presented her gifts of flowers and T-shirts, she kissed both boys. There were hoots and howls.

"If I'm an honorary mother I can do that," she said in her most motherly tone of voice.