A referendum may be ordered (except as to laws necessary for the immediate preservation of the public peace, health or safety, and laws making appropriations for the current expenses of the state government, for the maintenance of state institutions and for the support of public schools) either by petitions signed by five percent of the legal voters in each of two-thirds of the congressional districts in the state, or by the general assembly, as other bills are enacted. Referendum petitions shall be filed with the secretary of state not more than ninety days after the final adjournment of the session of the general assembly which passed the bill on which the referendum is demanded.