The Secretary of the Interior shall not enter into any contract for the education of Indians unless the prospective contractor has submitted to, and has had approved by the Secretary of the Interior, an education plan, which plan, in the determination of the Secretary, contains educational objectives which adequately address the educational needs of the Indian students who are to be beneficiaries of the contract and assures that the contract is capable of meeting such objectives: Provided, That where students other than Indian students participate in such programs, money expended under such contract shall be prorated to cover the participation of only the Indian students