Boards of supervisors are hereby authorized, in their discretion, to levy on the taxable property in the county at the time the annual tax levy is made, a tax for the building, repair, and equipment of agricultural high schools, as well as for the purchase of suitable lands or buildings, or both, or for the payment of debts heretofore created for any of said purposes, not to exceed three mills. Within twenty (20) days after levy has been made, twenty percent (20%) of the qualified electors of said county may file with the clerk of the board of supervisors a petition asking that the tax be not levied; thereupon, the question shall be submitted to an election of the people, which election shall be decisive to the levy for that year only. The maximum levy for maintenance purposes as fixed by the law for the agricultural high school, shall not be deemed a limitation on the tax levy for the building, repair, and equipment of agricultural high schools hereinbefore mentioned, nor the levy under this section, a limitation on the levy for maintenance purposes.