Section 44. The appropriate city or town retirement board, established under section twenty, upon the recommendation of the school committee, official, board or other body having control of janitors employed in the public schools of any city or town which has accepted this section or corresponding provisions of earlier laws, may retire, with an annual pension, any janitor so employed who has reached the age of sixty, after completing a service of not less than twenty-five years, and is physically incapacitated, and any janitor so employed, who has completed a service of not less than fifteen years, and is physically incapacitated by reason of injury received in the performance of his duties for such city or town. The pension shall be one half the compensation to which the pensioner would have been entitled for full employment during the last year of his service, but not more than seven hundred and fifty dollars a year, and shall be payable out of the appropriation for the support of public schools, maintenance of school buildings or payment of janitors.

No school janitor whose employment begins after June thirtieth, nineteen hundred and thirty-seven, shall be subject to the provisions of this section.