Unless other provision is made in the articles of incorporation the number of trustees shall be reduced by the grand lodge, assembly, conference, or other legislative or representative head of the colleges or institutions of higher education. At any annual session of the grand lodge, assembly, conference, or other legislative or representative head, there shall be dropped from the number of trustees to be elected at that session such a number of trustees as those present at the session determine, except that at no time shall the number of trustees composing the board be less than 15.