June 12, 1981

Several thousand students of Christian schools came by bus and plane from across North America to march in New York City yesterday. For two hours, to the thump and brass of such selections as ''Onward, Christian Soldiers'' and ''The Marine Corps Hymn,'' the youngsters and their teachers paraded down Central Park West, some in coats and ties. 'We thought it was going to be terrible,'' said Druman Heron, 17 years old, who came with eight schoolmates on a yellow bus from the Tecumseh Christian Academy School in Tecumseh, Okla. ''We thought people would be throwing eggs and water balloons.''

The Polish Communist Party has now defied Moscow and runs the risk of being branded a renegade in the Soviet bloc. By keeping Stanislaw Kania as party leader, the Central Committee$ in effect, turned its back on a warning from the Soviet Union that a new leadership must be installed to halt the course that Poland was following. ''Yesterday, it looked like the Politburo was not afraid of Russia, and today it looks like the Central Commuttee isn't either,'' a Polish diplomat said. He was summing up a two-day Central Committee meeting that ended early today with Mr. Kania still in power despite a campaign to oust him by hard-liners led by Tadeusz Grabski, a Politburo member.

IMPROVISATIONAL comedy is essentially impudent madness. Because virtually all of it is created at the moment, often out of current experience and usually at the suggestion of audiences, it has little time for such niceties as taste and taboos. Improvisational comics must be fast on their feet and quick with their tongues. Inventiveness and situations count, not gags. As a genre, modern improvisational comedy grew out of the intellectual ferment of the University of Chicago in 1955, and its golden age of comedic glory began in 1959 when the Second City improvisational group opened its own theater in a converted laundry. Some of its members came from such predecessors as the Compass Players and the Playwrights Theater Club. The movement was to spread in the 1960's from coast to coast, from the Committee in San Francisco to the Premise in New York, and it was to reshape American comedy.

Sales by the nation's retailers rose by two-tenths of 1 percent last month following a 2.1 percent decline in April that was the biggest dip in more than seven years, the Commerce Department reported today. The newly revised April decline, originally reported at 1 percent, resulted mostly from an 11.2 percent drop in automobile sales as spring rebate programs ended. In fact, the auto rebates have accounted for most of the recent changes in retail sales, the Commerce Department figures indicated. Excluding automotive products, the value of retail sales has remained virtually flat for the past two months.

All kinds of ''historical confetti'' have been tucked away in the New York Public Library on Fifth Avenue at 42d Street over the years, and a fair sample will be tossed out for viewing in Astor Hall at the main entrance starting today. Speakeasy cards, a leaflet declaring ''Save the 15-cent fare!'' Tom Thumb's greeting card and anti-Nazi literature from the German underground concealed in packets of tea are just a few of the items displayed in ''Typographical Small Wares: Ephemera From the Collections.'' Impossible to catalogue, the items are filed in scrapbooks and envelopes that are seldom exhibited. The earliest item in the display is a Roman Cathoic indulgence, sold by the Church in 1480 to laymen who wanted absolution for their sins. ISLE OF GREECE IN HEMPSTEAD A Grecian festival will transform the three-acre grounds of St. Paul's Greek Orthodox Church at 110 Cathedral Avenue in Hempstead, L.I., today through Sunday. Bouzouki music, folk dancing and all kinds of Greek food and wine will be offered. A taverna will serve full dinners with entertainment. Outdoor food sites will include a gyro and sausage pit. In a garden arbor, wines from various districts and islands will be offered, along with snacks such as spinach and cheese pies. Games, boutiques, a flea market and an art gallery are also planned. For travel and other information: (516) 483-5700.