Section 18. Any town may, and every town in which there are issued during any year certificates authorizing the employment of twenty or more persons who do not possess the educational qualifications enumerated in section one of chapter seventy-six, shall maintain for not less than forty evenings during the following school year an evening school or schools for the instruction of persons over fourteen years of age in orthography, reading, writing, the English language and grammar, geography, arithmetic, industrial drawing, both free hand and mechanical, the history of the United States, physiology and hygiene and good behavior. Such other subjects may be taught as the school committee considers expedient.