171.131.  Seventh and eighth grade pupils may be sent to another district — tuition, how paid. — The board of education of any elementary school district may elect to maintain an elementary school in the district for grades one through six only.  In that event tuition shall be paid by the sending district as provided in section 167.131 for pupils in grades nine through twelve.  Pupils in grades seven and eight shall have the option of attending any approved school in the county of residence or in an adjoining county.