Rivi Tadjer, United Nations delegate from the Kingdom of Saudi Arabia, was having a hard time keeping the Third World countries in line Saturday night. The debate about a Saudi-backed resolution for Palestinian autonomy had come down to one crucial procedural vote, and Rivi was busy working the aisles, buttonholing delegates and revising her head count.

"This is the big thing. We really need this one," Rivi said, wringing her hands. "Oh, please. . . "

While Rivi nervously went to talk with a wavering delegate, another Saudi, Habon Addou, said stoically, "She gets too excited about these things. She must remember to be "diplomatic."

When the vote finally came, the Saudis were defeated 22 to 32, with 19 absentions. Rivi, incredulous at the disunity of the Third World and the inacccuracy of her head count, threw down her placard in disgust. Habon remained impassive.

Rivi and Habon, students at Georgetown Day High School in Northwest, had just learned a lesson in diplomacy. It was the kind of lesson that doesn't come in textbooks -- the realization that in foreign affairs, policy is often the result of politics rather than substance, and backroom bargains frequently are more important than podium-pounding speeches.

Learning those lessons in a practical setting is one of the goals of Georgetown University's model United Nations project for high school students. Last weekend, about 2,400 students representing 195 high schools from across the country turned the ballrooms of the Shoreham Hotel into U.N. conference halls and caucus rooms for Georgetown's 18th North American Invitational Model United Nations.

The schools were assigned the roles of specific countries far enough in advance (as long ago as October) to give them time to research the history and foreign policies fo their countries. Georgetown University aranged for 120 embassies here to brief student groups to enable them to give a more accurate portrayal.

"They come away with an awareness of international relations," said Kelley S. Coyner, a student in Georgetown's Edmund A. Walsh School of Foreign Service, who played the coveted role of U.N. secretary general. "They learn a lot about what a country is like. If they are representing a Third World country, they learn the frustrations. They (also) learn that the U.N. does much more than what you read about in the newspapers."

Five Washington high schools were represented at the conference -- Georgetown Day, Georgetown Visitation Preparatory School, Woodrow Wilson, Sidwell Friends and St. John's College High School. Through the sights and sounds of the four-day event, each of the District schools took a commanding role and kept a high profile.

In the powerful Security Council, a Georgetown Day student had the superpower role of the People's Republic of China. In the Economic and Social Council, Georgetown Visitation was Ethiopia, at the center of the conflict in the Horn of Africa. And St. John's College was Democratic Kampuchea (Cambodia), the country in the storm's eye in Southeast Asia and the center of the debate over nonintervention and refugee relief.

In the Political and Security Committee, Rivi and Habon, as the Saudis, were elected bloc leaders by the Arab contingent, and took the lead in pushing the committee to its pro-Palestinian stance while keeping the Arab radicals in check. "Saudi Arabia is very, very conservative, but you have to be very, very agressive to score points with the judges," Rivi explained, "If you're prepared, you can get away with anything."

"We're all over the place," Habon said.

Across the room, Tim Hall, an 18-year-old senior from Woodrow Wilson, was staying decidedly in the middle as the delegate from Papua New Guinea. "I'm staying pretty neutral," he said. "It's a pretty neutral country. We're mostly interested in economic issues, and this is a political committee."

Meanwhile, Sidwell Friends was at the center of another controversy as El Salvador. In the Social, Cultural and Humanitarian Committee, the delegates representing the ruling Salvadoran junta managed to convince the other Latin American nations to choose them as bloc leaders.

"Our most improtant thing," said 17-year-old senior Liz Singer, the Salvadoran chief of mission, "is noninterference from the superpowers, but we'll take American arms." Singer said her delegation had a briefing the day before at the El Savadoran embassy, where she heard government charges that Salvadoran guerrillas were being aided by Nicaragua.

"I just made a really violent speech against Nicaragua," Liz said. "I'm having a great time."