The number of reported crimes on college campuses has dropped in the past two decades, but there has been a significant increase in sexual assaults, according to a report released on Wednesday. The report, “Indicators of School Crime and Safety 2015,” is the latest edition of an annual report that provides insight into safety problems at elementary, secondary, and postsecondary institutions around the country. It was prepared by the National Center for Education Statistics, the Bureau of Justice Statistics, and the American Institutes for Research.