A school board shall not issue bonds, except for the purpose of refunding outstanding bonds, in an amount which would cause the aggregate amount of bonded debt of the district, less the amount of sinking funds on hand for the payment of such bonded debt, to exceed 10% of the assessed value of the real property in the district or in the case of Sussex County school districts, 10% of 50% of the full market value of real estate, in Kent County school districts, 10% of 60% of the full market value of real estate; and in New Castle County school districts, 10% of 100% of the full market value of real estate, whichever is greater. For purposes of this section, the full market value of real estate shall be determined by the Assessment to Sales Ratio Study conducted annually by the Office of Management and Budget.