AS Tommie Leaders, 22, approached college graduation last spring, his professors told him he would have no trouble getting hired. “You’re a guy teaching elementary, ” they said. Mr. Leaders, who earned his education degree from the University of Nebraska in June, started teaching fifth grade last month in Council Bluffs, Iowa. He is the only male teacher in the building. Across the country, teaching is an overwhelmingly female profession, and in fact has become more so over time. More than three-quarters of all teachers in kindergarten through high school are women, according to Education Department data, up from about...