In order to make provision for the proper preservation and application of donations from private sources by gift, devise or otherwise, heretofore made to the board of trustees of the Alabama Boys Industrial School, the board of trustees of the Alabama Training School for Girls or the board of trustees of the Alabama Industrial School for the uses and purposes intended by the private donors and in order to encourage future donations from private sources by way of gift, devise or otherwise to said schools and assure prospective private donors of the use thereof at the particular school or schools designated as the object of donations and to prohibit the diversion of past and future donations to said schools from the uses and purposes for which the same were made, the advisory board of each school is authorized and empowered to serve as trustee or trustees for an endowment trust fund for its respective school set up under an agreement with a bank or banks organized either under the national banking laws or the banking laws of this state and having trust powers, to accept donations from private sources for the benefit of the particular school involved, manage the trust property in a prudent manner in accordance with sound financial principles and pay out so much of the income therefrom and/or of the principal as may be required by appropriate resolutions adopted and approved by the youth services board and, in the event of disestablishment of the particular school, to provide for the termination of its endowment trust fund and transfer of trust property then on hand to the department of youth services for use for the particular uses and purposes of each separate endowment fund then included in the trust, or, if such use has ceased to be practicable, then for such use as in the department's judgment constitutes an equitable approximation of such uses and purposes. The trustee or trustees of any endowment trust fund established pursuant to this section shall periodically, not less than once every three years, make a full accounting of its handling of the trust estate to the youth services board, and written approval of the trustee's or trustees' accounts by said board shall be final and binding and have the same full force and effect as a partial final settlement or final settlement, as the case may be, had the accounting been accomplished through judicial proceedings. The advisory board of said school is prohibited from authorizing or directing any payment out of the endowment trust fund of said school for any purposes contrary to the expressed uses and purposes of the private donors of a donation constituting a part of the school's endowment trust fund.