The special school district shall provide free vocational instruction for children under the age of twenty-one years resident within the district’s boundaries. The vocational program of instruction shall be approved by the state department of elementary and secondary education and shall be so designed as to provide sufficient vocational and academic training for the student to receive a high school diploma at the completion of the twelfth grade. The board of education of the special school district, subject to the approval of the state department of elementary and secondary education, shall establish standards for admission to vocational programs operated by the district.