No new school district shall be planned by an area education agency board nor shall any proposal for creation or enlargement of any school district be approved by an area education agency board or submitted to electors unless there reside within the proposed limits of such district at least three hundred persons of school age who were enrolled in public schools in the preceding school year. Provided, however, that the director of the department of education shall have authority to grant permission to an area education agency board to approve the formation or enlargement of a school district containing a lower school enrollment than required in this section on the written request of such area education agency board if such request is accompanied by evidence tending to show that sparsity of population, natural barriers or other good reason makes it impracticable to meet the school enrollment requirement.