One or more public evening schools, in which attendance shall be free for persons resident in the city or town in which the school shall be located, and in which the speaking, reading, and writing of the English language shall be taught for two (2) hours on each of at least one hundred (100) nights between the first of September and the first of June in each year, shall be established and maintained by the school committee of every city or town in which twenty (20) or more persons more than sixteen (16) and less than twenty-one (21) years of age, who cannot speak, read, and write the English language, are resident; provided, that the school committee of two (2) adjoining cities or towns may unite for the purpose of establishing and maintaining jointly, at some convenient place, an evening school for persons resident in these cities or towns.