(Reuters) -A Myanmar military airstrike on a school last week killed at least 19 students, an ethnic militia said, as the war-torn nation's ruling junta steps up a campaign to retake territory ahead of a planned election in December. Children were killed and injured at the boarding school in Kyauktaw Township in Myanmar's restive Rakhine state, the U.N. children's agency UNICEF said in a statement. It did not say how many were killed in the incident, but the Arakan Army, a militia battling the military in the western state, said at least 19, aged between 15 and 21, had died.