236.55  Interest and sinking funds may be invested in certain bonds, warrants, and notes.--The school board shall have the power at all times to invest the interest and sinking funds collected for the retirement of any bonds of the school district in any investment as authorized in s. 236.49(2). The school board shall have authority at any time to use the interest and sinking fund of any district for purchasing, for the purpose of canceling and retiring, bonds outstanding against the interest and sinking fund of said district at any price which will result in a net saving to the taxpayers of the district; provided, always, that the school board shall have the right to keep the interest and sinking fund on deposit earning the rate of interest agreed upon until such time as within its judgment it may be able to invest it in bonds, warrants, or notes to better advantage as herein provided for.