The Board of Education shall have the power to create and incur indebtedness against the District and issue bonds to secure the same for the purposes and payable as provided by law, but no debt shall be incurred or increased which will cause the aggregate indebtedness to exceed at any one time five (5) per cent of the assessed value of the taxable property within the District without the consent of the electors thereof at a public election in such manner as shall be provided by law, nor shall the aggregate indebtedness of the District exceed fifteen (15) per cent of the last assessed valuation of property taxable for school purposes in the District.