Except in school districts of the first class, the board of school directors of every school district shall employ for its high school, during the entire term, a sufficient number of teachers for the teaching of any of the subjects included in the program of studies prescribed by the standards of the State Board of Education for which there shall be an application by fifteen pupils belonging to the grade in which said subject is specified in the program of studies. No pupil shall be counted among the applicants for such subject unless, in the judgment of the principal of the school, he is both qualified and entitled to take such subject in the curriculum he is pursuing. No teacher shall be employed to teach any branch other than those enumerated in his certificate.