CULEBRA, Puerto Rico — When temperatures soar at a public school on the small Puerto Rican island of Culebra, students scatter in search of relief. During a summer program last month, as outdoor temperatures climbed into the upper 80s, Zedrik, a rising third grader, sat in the schoolyard beside a bush not tall enough to shade him entirely from the sun. Kenneth, a ninth grader, took refuge in the library. Nayla, an eighth grader, went to the director's office, where an air conditioner offered limited relief.

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