Any money in the hands of any treasurer as a sinking fund for the redemption of bonds which are a valid and legal obligation of the school district to which such money belongs or for the payment of interest on any such bonds, and which is not currently required to retire bonds and pay interest on bonds, shall be invested by the treasurer, when so ordered by the school board or board of education, (1) in bonds, treasury bills or notes of the United States, or (2) in interest-bearing time certificates of deposit in depositories approved and authorized to receive county money, but in no greater amount in any such depository than the same is authorized to receive deposits of county funds. The interest earned on such investments shall be credited to the sinking fund from which the invested funds were drawn.