Section 24. Smith Agricultural School, established under chapter one hundred and fifty-one of the Special Acts of nineteen hundred and eighteen, shall be maintained by the city of Northampton as a state-aided approved vocational-technical school under and subject to sections one to twenty-two, inclusive; provided, however, that the superintendents of said school shall consist of the mayor and superintendent of schools of said city, ex officiis, and three other superintendents to be elected at its city election by ballot, as provided in the will of Oliver Smith, and that said superintendents shall have the powers of local trustees elected under section three. Upon the death or resignation of an elected superintendent of said school his successor shall be chosen by the city council and the remaining superintendents in joint convention, who shall serve until the next city election.