With his head bowed to the keys of the East Room Steinway, ignoring the Park Service employes arranging boxes of shrubbery around him, Andre-Michel Schub practiced yesterday afternoon for his White House debut.

As he moved spiritedly through Mendelssohn's "Variations se'rieuses" (Op. 54), Schub seemed the model of rapt concentration. His eyebrows reached high above his eyeglasses and the sweat poured down his face, seemingly a refresher, not a nuisance.

Schub, 28, who won the sixth Van Cliburn International Piano Competition last June and played for last night's state dinner in honor of Israeli Prime Minister Menachem Begin, said he wasn't especially nervous. "Not any more than another time," he said. "This is a great honor as far as the presidency of the U.S. and to play for the prime minister of the great State of Israel."

The Reagan White House, said Schub, had requested some Chopin. "Everyone loves Chopin," said Schub, who made some last minute adjustments in his program. He selected Scherzo No. 4 in E Major (Op. 54), as well as a composition of Liszt's, "Etudes d'Exe'cution Transcendante d'apre s Paganini." "The Liszt is vibrant. The Mendelssohn is as close to any Israeli composer that I play."

Since he won the Cliburn honor, Schub, who was born in Paris but grew up in Brooklyn, N.Y., has had a vigorous touring schedule. The highlight so far, he said, was his appearance with Mstislav Rostropovich at the Kennedy Center last month. "I guess you could say I idolize him. He's a real musician," said Schub.

But the pianist declined to describe any political favorites. "I have my own personal politics but I don't feel they enter my public forum," said Schub.