The cost of administering unsold school lands shall include expenditures necessary for developing such school lands to their most productive use. All expenditures shall be directed by the board in exercising its responsibility as a trustee over the school land trust and shall not exceed twenty percent of all rental and bonus payments of the previous year, which amount shall be appropriated for each year to the board for payment of development and improvement costs completed and approved by the board upon contracts let for this purpose. All rental and bonus payment deductions as authorized by this section shall be remitted to the State Treasurer for credit to the Land Improvement Fund, which fund is hereby created, to be expended upon proper warrants in accordance with law. Any money in the fund available for investment shall be invested by the state investment officer pursuant to the Nebraska Capital Expansion Act and the Nebraska State Funds Investment Act.