Nothing in this constitution contained shall forbid the formation of districts for school purposes situate in more than one county or the issuance of bonds by such districts under such general laws as have been or may hereafter be prescribed by the legislature; and the officers mentioned in such laws shall be authorized to levy and assess such taxes and perform all such other acts as may be prescribed therein for the purpose of paying such bonds and carrying out the other powers conferred upon such districts; provided, that all such bonds shall be issued subject to the limitations prescribed in section eighteen of article eleven hereof.