As a gourmand of elementary school cafeteria cuisine, Secretary of Agriculture John R. Block failed yesterday on several counts.

He skipped the cole slaw, the only "green veggie" of the day at Mount Vernon Elementary School in Alexandria. He flouted preteen tradition by squashing neither his barbecue bun nor his ice cream sandwich so he could eat the filling first.

On the other hand, Block did exhibit a wicked knack for taking the paper off his straw using one hand and his teeth.

"He wants to eat here? Oooohhnnoooo," gasped fifth-grader Joanne Fenton, clapping her hand to her forehead and turning away in mock horror as Block joined students in the school cafeteria.

"If he's from the government, tell him not to eat it," grimaced tablemate Naomi Coon, 11. "It'll kill him."

It was Block's first hand-to-mouth experience in the Washington area with his department's school lunch program, which along with the food stamp system accounts for most of the 95 million meals a day that the USDA subsidizes.

"Did you get your money's worth?" Block was asked afterward. "He was a guest of the school," interposed principal Walter Krug.

"Obviously, it was a bargain," said Block, laughing.

Block's barbecue lunch ("What else for an old hog farmer?") roused all the hooplah of a state dinner. The table where Block and Krug sat with a handful of fourth-graders was walled in by cameramen, reporters, boom microphones, studio lights and Secret Service agents. Most of the other students in the cafeteria seemed far more interested in Block's entourage than with the secretary himself.

"Ask that lady to come over here," demanded Jermaine Wilson, 11, pointing to a woman carrying a television camera. "I wanna be on television."

"Are you nervous?" asked fourth-grader Juliet Wilson, sitting across the table.

"A little, with all these people," nodded Block. "Are you?"

"What do you do all day?" quizzed Killian Cousins, president of the student body, and "What's in the Department of Agriculture?"

"I answered their questions as well as I could," Block said later, "and, when I couldn't, I faked it."

Mount Vernon, at 2601 Commonwealth Ave. in a working-class neighborhood, was chosen for Block's visit because a high percentage of the students participate in the lunch program. Of some 800 students, about 40 percent eat lunches free or at reduced rates. The full charge is 85 cents; a third of the students are eligible to eat free under government guidelines, and another 8 percent pay only 40 cents each.

USDA not only subsidizes the school program directly, through cash reimbursements, but donates an estimated 19 cents' worth of food per meal. Last year, the Alexandria schools used more than $200,000 worth of USDA commodities.

Block was kidded at the school about a short-lived USDA proposal last year to let ketchup count as a school lunch "vegetable."

"There might have been ketchup in the barbecue sauce," Block said good-naturedly, "but they didn't tell me it was a vegetable."