Pacing the front of his Prince George's County classroom, teacher William Yoder tried to draw out the lessons of Watergate from teen-agers who were in diapers when the burglars entered Democratic Party headquarters 15 years ago today.

Yoder asked the 21 juniors in his Largo High School American history course -- most of them from the black middle-class neighborhood nearby -- to assess the impact of Richard Nixon. "What did he destroy?"

"Trust . . . in the government," replied a girl in black denim.

Twenty-five miles away and a day earlier, West Springfield High School teacher Dennis Patrick taught a lesson on the same subject, issuing a challenge to his 11th-grade advanced placement class at the mainly white school in one of Fairfax County's more affluent Republican neighborhoods.

"As a citizen of this country, how much do you think you have the right to know?" he asked.

The students argued fiercely about the investigative journalism that helped propel Nixon from the presidency. "They were blowing everything open so the president had to concentrate on that," complained a blond girl with glasses. "Another country could have come along and blown us away."

The Watergate lessons in these two classrooms last week were shaped by the differing styles of the teachers and the backgrounds of their students, but they shared some common themes.

In each, the teacher drew on this year's 200th anniversary of the U.S. Constitution to pose the question of whether the democratic system worked to right wrongs. Each also noted parallels between the Watergate affair and this spring's hearings on the Iran-contra dealings. And each tried to convince his students that the duties of a responsible citizen include an aggressive search to become well-informed.

As they stood in front of their teen-aged students, the two men -- one 31, the other 42 -- recalled their own amazement and anger at the events that brought down a president and helped mold the attitudes of their generation. Each struggled to make Watergate come alive for teen-agers who often view the event as irrelevant.

"They're no more interested in the issue than in the impeachment trial of Andrew Johnson 100 years earlier," said Dennis Pfennig, history department chairman at Hayfield Secondary School in Fairfax County.

"It's history; it's gone and it's passed," West Springfield junior Stefanie Gooden, 16, said during lunch in the school cafeteria. "It doesn't really affect me."

For Yoder, who was a conservative high school student in central Pennsylvania at the time of the break-in, the events that followed made an indelible imprint.

"A lot of myths that I had about government were shattered," he said in an interview. "That, more than anything else, colors the way I teach history. I feel my job is to walk around with a big pin and burst a lot of bubbles. A lot of these kids have a very shaky concept of reality."

During class that day, Yoder tried to convey to his students the electric uncertainty of living through the break-in, congressional hearings, Saturday Night Massacre firings, the resignations that brought Gerald Ford to the presidency and Nelson Rockefeller to the vice presidency.

"People your age were very upset," he told his students. "We had no idea what was going to happen. There were two people in office who people hadn't voted for."

His own family argued about Watergate around the dinner table, the teacher said. He described high school classes that turned into political arguments among best friends with differing views.

Yoder carefully led his students through the events of 1972 and up to Nixon's resignation two years later, extracting a lesson from each turn of history. Watergate was an example of the power struggle between the president and Congress, he said -- a struggle that is now being repeated during the Iran-contra hearings.

Was Nixon the only president who deliberately did something wrong? Yoder asked. He elicited from his students examples from other eras, but contributed his own view that Nixon had a more profound impact in destroying trust in government.

"As a citizen, with Watergate or Iran, what is your responsibility?" the teacher queried.

A youth in a striped shirt replied: "To talk to your senator, or something, let them know what you feel."

It also is important to keep informed, Yoder told his students. They would never write a research paper based on only one book, he reminded them. (Several students grinned.)

If Yoder taught like an earnest trial lawyer stacking blocks to build his case, Dennis Patrick resembled a machine gunner spraying ideas like bullets around his classroom.

Who will check the excesses of government if we do not have a free press? Did the system prevail in the Watergate affair, or was it something else -- blind luck, for example? And how is it, he asked, that 15 years after Watergate, the United States is "embroiled in it again"?

Patrick was teaching at a Fairfax County high school during the day and umpiring evening softball games throughout the Watergate affair; he ran home in between his two obligations to catch what he could of the televised congressional hearings. "This is the point I really want to emphasize: These types of situations cannot take place if the public is vigilant," he said of the lesson he hopes his students will take with them.

His 22 third-period students reached no agreement on answers to questions he posed, but they honed their debating skills by weighing issues echoed in today's headlines.

"There's a lot of things the government has to be able to do behind closed doors," argued a girl in a pink sweater and white slacks, who said her father's job involves handling classified documents. Americans have a right to know about events that affect them directly, she said, but do they really need to know about covert bases in Honduras?

"It's your tax dollars," another student shot back.

In Patrick's second-period class, the students concluded that loyalty to Nixon helped produce some wrongs that the perpetrators thought were committed for good reasons. That led them to a discussion of whether Jimmy Carter was elected as a repudiation of what was viewed as Republican immorality -- and whether the 1988 elections will be similarly affected, in light of the Iran-contra revelations.

Patrick closed the class with a question: Will there be another scandal 15 years from now?

The shout was unanimous: "Yes!"