The board of education of each school district may cause an annual census to be taken of all children under the age of twenty-one years residing in the district as of the first day of January. Such census may show separately the number of children under the age of five years, the number between the ages of five and twenty-one, and the date of birth of all such children and such other information relating to such children as may be provided for by law or under rules and regulations adopted by the state board of education. Such information may be ascertained by means of personal interviews with the families of children living in the district. Boards of education may use any other reasonable method of ascertaining such information.