Section 5A. Whenever the board or officer having charge of the public parks in a city or town having a population of more than fifty thousand determines to lease concessions in any such park, said board or officer shall invite sealed proposals for such leasing which shall be publicly opened and read and shall award the contract for any such concession to the highest responsible bidder who, in the opinion of said board or officer, would be the most suitable from the standpoint of character, reputation and experience to operate such concession, provided, however, that the provisions of this section shall not apply to an award of such a lease to a nonprofit organization or group organized for the purpose of promoting the assistance of youth programs.