On some days, Shawn does attend classes with about 10 other students who are also behind. On many other days, he simply does not bother. He likes math, but the algebra course he has been forced to take repeatedly is too easy, he said, so he does not make an effort. “The sad thing is, some of the kids can’t even do it,” he said.

Murrow, a 4,000-student school in the Midwood neighborhood with a far-reaching variety of course offerings that are reminiscent of a small liberal arts college, was founded in 1974, and it gives its students considerable freedom. Periods are called bands. There are no bells, and no one is herded from class to class. Free time is scheduled into every school day, and students can choose to eat, to sleep, to do homework, to do nothing or, as Shawn has often done, to play cards in the cafeteria.

“It is a school where if you don’t have your personal responsibility together, you could drop out,” Shawn said.

Ms. Siegel-Graf, the assistant vice principal, said Shawn was allowed to accompany his teammates on the plane to Missouri on Wednesday afternoon after a conference at which he promised that, this time, he would begin going to school regularly. Shawn turns 17 on April 24  11 days after the nationals start  and Ms. Siegel-Graf said Shawn and the school had worked out an arrangement in which although he would still be technically enrolled at Murrow, he would begin taking courses to prepare for the G.E.D diploma.

The rules for the national tournament require students to be enrolled full time in school in the United States or its territories for the entire semester. They also state, “The coach is responsible for assuring that all of his players are properly registered and eligible to participate as members of his team.”

On a recent Thursday, a few weeks before the nationals, Shawn said he had not gone to school because he had a sore throat. Later, he said he had run out of minutes on his mobile phone and needed to win some money playing chess to pay the bill.

Here, among the businesspeople and tourists on Wall Street, Shawn sticks out with his Yankees cap, baggy jeans and well-worn red and black Nike high tops, but he also mixes easily with the stockbrokers and others who come to play.