Whenever a district has been authorized to levy Class A taxes, the board may borrow money in anticipation of the collection of such taxes for which a levy has been made to the extent of ninety percent thereof, and may issue negotiable warrants to evidence such loans payable in not more than twelve months from the date thereof, and bearing interest at a rate not to exceed six percent per annum. Such warrants may be issued and sold in such manner as the board may determine.