Ed. Code, § 15740

The Controller shall determine and maintain a record of the amount due the state in connection with each apportionment made to each grade level of a district under this chapter. He or she shall compute interest on the original amount of the apportionment at the rate fixed by the board, from the date of issuance of the Controller’s warrant covering the payment to the county treasurer of any portion of the apportionment until the first day of July of the fiscal year next succeeding that in which the warrant was issued. Thereafter, interest shall accrue to and be compounded as a part of the principal amount due the state pursuant to the apportionment through the 30th day of the following June of each year, until the principal and interest have been paid, or until the interest ceases to accrue, as provided in this chapter. Interest on unpaid school building aid apportionments shall be computed as if the annual repayment were credited on the first day of July of the fiscal year in which the repayment is withheld.