Section 43. The appropriate city or town retirement board, established under section twenty, upon the recommendation of the school committee of any such city or town may retire from active service and place upon the pension roll any teacher of such city or town who is sixty or over or who, in the judgment of said committee, is incapacitated for useful service, and who has faithfully served such city or town for twenty-five years. The amount of the annual pension allowed to any person under this and the preceding section shall not exceed one half of the annual compensation received by such person at the time of such retirement, and in no case shall it exceed twelve hundred dollars.