In all counties of the first class except counties of the first class not having a charter form of government, any board of education, other than boards in urban districts, in charge of a public school system maintaining a classified high school, previously approved by the state board of education, and employing a superintendent devoting his full time to supervisory and administrative work, may employ and enter into contract with a superintendent of schools for the school district for a period of not to exceed three years. This law shall not invalidate or repeal any other law of this state relating to the employment of teachers, principals or superintendents of public schools.