"Good afternoon, fellow patriots of the United States of America," the speaker declared, as 250 students in the elementary school auditorium listened carefully. "I am here to speak about your future. I believe America is ready for change. . . . "

A rival speaker, dressed in a bright plaid jacket, responded later, "America is back. America is strong. The economy is at its peak. Thank you for your support."

At Murch Elementary School in Northwest Washington yesterday, the debaters tried to sound like Walter Mondale and Ronald Reagan. The two sixth graders at the lectern even tried to look their parts, though the boy who played Mondale, Todd Johnson, said the Ronald Reagan pompadour of his rival, Jonathan Knight, was much neater in the morning before the students' heads were checked for lice.

The debate at Murch, which also included surrogates for Geraldine Ferraro and George Bush, was part of an effort by the District public school system to teach children about the Nov. 6 presidential election. On Nov. 1, students throughout the city will cast ballots for Mondale or Reagan. So will thousands of other students in the Maryland and Virginia suburbs and around the country, as a part of the 1984 National Student/Parent Mock Election.

The nationwide results will be tabulated that evening in Fort Worth.

"We're trying to get the excitement of the election into the classroom," said Gloria Kirshner, head of the mock election project, which was launched last spring and has been endorsed by both major parties.

Kirshner, a former teacher, now is vice president of a New York City firm that prepares classroom guides to television. "We want to get students involved instead of watching the election go by around them," she explained. "You don't just emerge from the egg a solid citizen. You have to be taught."

Local preelection activities yesterday also included a debate before about 700 student government leaders between adult representatives of the two candidates -- lawyer Shelly Todd, media coordinator for the D.C. Reagan-Bush campaign, and Reginald Petty, a former Peace Corps director who works for the Mondale campaign. That encounter at Fletcher-Johnson School in Southeast Washington was much more subdued than the one at Murch, where the students did the talking for the candidates.

"I think the American people still want to help other people," said Johnson, the 11-year-old Mondale. "I don't believe in crime, but I am sympathetic to Robin Hood. He carried it to the extreme, but I believe in taxing the rich and distributing it to the poor."

Erin Toland, an 11-year old girl in preppy-like knee socks, red skirt and vest, stood in for Bush. "We have brought down inflation," she declared. "We have cut taxes 25 percent. That gives every American more money to spend, save, and invest."

The stand-in for Ferraro, Julie Lanoff, said, "I am here to prove that this is not a man's world, which it will be if my opponent is elected." She strongly endorsed the Equal Rights Amendment. "Women should not have to stay at home," she said, "unless they want to."

Applause for all the candidates was about equal. It was kept that way by a boy near the stage who held up a sign telling the audience when to applaud and when to be quiet.

Roseanne Faust, the school's enrichment coordinator, said it was somewhat difficult keeping the event bipartisan because Murch students, like D.C. voters, are heavily Democratic. "We twisted arms to get people to be Republicans," she said.

Knight, who portrayed Reagan, said he actually prefers Mondale but decided "it would be a good experience for me" to speak for the president.

The results are already in from the first presidential preference poll of D.C. students, which was conducted by The Weekly Reader Magazine. Mondale carried the District "handily," the magazine said. But in its nationwide poll, with about 870,000 students voting, it said the Democratic candidate was ahead in only one state -- Arkansas.

Reagan's overall support in the Weekly Reader poll was 64 percent to 33 percent for Mondale. The magazine said that since it started polling in 1956, it has predicted every election correctly except for 1968.