A school district or county office of education may enter into contracts, including multiple year contracts, with private sector individuals, businesses, and other entities for the purpose of receiving revenues to supplement its schoolbus emissions reduction fund in exchange for the issuance to the private sector contributor of emission reduction credits resulting from the purchase by the school district or county office of education of low- or zero-emission schoolbuses or the retrofit of existing schoolbuses. If there are multiple private sector contributors, each of those contributors shall receive a share of the credits allocated in proportion to their contribution, as specified by the school district or county office of education at the time that the parties enter into the agreement.