Ed. Code, § 15753

Any portion of an apportionment paid to a school district under this chapter shall be available for expenditure by its governing board for not less than one year nor more than three years, as the board shall determine, after the date on which the warrant covering the portion of the apportionment was issued by the Controller. For the purposes of this chapter, an apportionment shall be deemed to be expended at the time and to the extent that the amount thereof on deposit in the county treasury has been encumbered by the creation of a valid obligation on the part of the school district. Upon the expiration of its period of availability, the unencumbered balance of any apportionment made under this chapter shall become due and payable to the State of California; and the governing board of the school district and the county treasurer shall pay the amount of the unencumbered balance to the Treasurer, out of the funds, and in the manner specified in Section 15752. The payment shall, on order of the Controller, be deposited in the Public School Building Loan Fund in the State Treasury.

It shall be the duty of the governing body and county treasurer to make the payments to the Treasurer as provided in this section, and it shall be the duty of the Controller to enforce the collection on behalf of the state.