Mass. Gen. Laws ch. 71 § 7B
To provide for the reimbursement of part of the cost not reimbursable under section seven A incurred directly by a school department or indirectly by a city or town on account of an assessment made to meet the cost of maintaining a public transportation system used for the transportation of pupils in a city or town using public transportation facilities licensed under the provisions of sections one and seven of chapter one hundred and fifty-nine A or operated under the provisions of chapter one hundred and sixty-one A or chapter one hundred and sixty-one B for the transporting of pupils to and from school who reside more than one and one half miles from the school they attend as determined by the commissioner, or, if the determination of the number of pupils residing more than one and one half miles from the school of attendance is impracticable, the commissioner shall make such determination according to the following formula: From the net average membership in the public and private schools, determined as provided in section five of chapter seventy, the commissioner shall estimate the number of pupils who reside more than one and one half miles from the school they attend, which number shall not exceed ten per cent of said net average membership.
The commissioner shall on or before November first of each year, certify to the commissioner of revenue a transportation allowance payable to such city or town for the preceding school year, the amount of such allowance to be determined by him by multiplying the number of pupils or the estimated number of pupils residing more than one and one half miles from the school by the average cost per pupil in the city or town for transporting all such pupils; provided that such cost shall not exceed twenty cents per pupil per day.
Funds received by each city or town shall be used to pay the cost of providing public transportation or to reimburse a city or town for payments on account of any assessment made upon it to meet the cost of maintaining a public transportation system, provided that notwithstanding the provisions of this section, no amount shall be approved as a reimbursement by the commissioner as herein provided in excess of the amount to be paid by such a city or town for said public transportation.
Mass. Gen. Laws ch. 71, § 7B