A chemistry teacher at Manhattan’s award-winning Beacon High School set two 10th graders on fire Thursday morning — leaving one boy with serious burns — in a fiery experiment gone horribly awry. Teacher Anna Poole had hoped to treat her students to a “fun” demonstration during their first class back from the holiday break, students told The Post. But four flaming crucibles in the Upper West Side public school’s third floor chemistry lab instead exploded into a fireball that raged across a countertop and engulfed sophomore Alonzo Yanes, 16.