THE HIGH SCHOOL civics students who wandred into the House Foreign Affairs subcommittee hearing on the border war in Yemen last week might be forgiven if they went back to their classrooms confused.

To begin with, the maps in the Rayburn Building hearing room clearly showed that North Yemen is actually west of South Yemen, which is east, and in some places north, of its centuries-old rival, North Yemen.

The next problem, as Rep. Ben Rosenthal (D-N.Y.) put it, was to distinguish "the good guys from the bad guys."

For the moment, the "good guys" are the North Yemenis, known formally as the Yemen Arab Republic. The "bad guys" are the South Yemenis, or the People's Democratic Republic of Yemen, whose Marxist-leaning government has welcomed advisers from the Soviet Union, Cuba, East Germany and Ethiopia.

It hasn't always been so clear-cut. Last summer, for instance, Rep. Paul Findley (R-Ill.) wrote an article for The Washington Post urging the Carter administration to renew diplomatic relations with South Yemen. "South Yemen is vital to our security," Findley wrote. But at Monday's hearing it was Findley, the subcommittee's ranking Republican, who was the staunchest defender of President Carter's decision to send arms to North Yemen.

Subcommittee chairman Lee Hamilton, an Indiana Democrat, was bothered by the admission of State Department witness William R. Crawford that the North Yemenis have no pilots qualified to fly the dozen F5E jets and no commanders trained to operate the 64 M60 tanks that are the heart of the arms package.

"What was the hurry?" Hamilton demanded.

The presence of the jets would be "largely symbolic," Crawford, a deputy assistant secretary, mumbled mellifluously. As for the tanks, Crawford said, the Yemenis could learn to operate them in a couple of weeks, or at least in a couple of months. "The Yemenis," Crawford, a former ambassador to North Yemen, assured the congressmen, "are well know for their manual dexterity."

Hamilton, not totally reassured, wondered how the tanks and the 100 armored personnel carriers could maneuver on the desert country's rocky, mountainous terrain.

"They would have to stay on the main roads," Crawford replied, noting that what passes for highways in North Yemen are dirt roads.

"You'll have a traffic jam," said Hamilton. "We might be better off sending them traffic lights."

It's not so much that North Yemen itself is important to the United States (there isn't a drop of oil in either Yemen) but, Crawford noted, the North Yemenis are "friends of friends" -- the Saudi Arabians.

Then why don't the Saudis supply the planes and tanks, asked freshman Rep. Michael Barnes (D-Md.)

The Saudis, Crawford explained, "prefer a softly modulated response." This diplomatic language turned out to mean that they are willing to pay for the arms but don't want to be out front about it.

"Are we prepared to go to war to protect Saudi Arabia?" pressed Hamilton.

Crawford answered in words that Rep. Gerry Studds (D-Mass.) called "diplomatic mumbletypeg" but that Hamilton said "I interpret to be in the affirmative."

That was correct, Crawford said.

"This has a frighteningly familiar ring to it," said Studds. "Let's get out of here before we scare all of us to death."