The proceeds of refunding bonds shall either be immediately applied to the retirement of the bonds to be refunded or be placed in escrow to be applied to the payment of the bonds upon their presentation therefor; provided, however, any accrued interest and any premium appertaining to a sale of refunding bonds may be applied to the payment of the interest theron [thereon] and the principal thereof, or both interest and principal, or may be deposited in a reserve therefor, as the board may determine. The escrow shall not necessarily be limited to refunding bond proceeds but may include other moneys made available for said purpose. Any escrowed proceeds pending such use, may be invested or reinvested in federal securities. Such escrowed proceeds and investments, together with any interest to be derived from any such investment, shall be in an amount at all times sufficient as to principal, interest, any prior redemption premium due, and any charges of the escrow agent payable therefrom, to pay the bonds refunded as they become due at their respective maturities or due at designated prior redemption date or dates upon which the board shall exercise a prior redemption option.