Cal. Code Regs. tit. 5, § 15321 – Replacement.

(a) Replacement is considered completed when the replaced bus has been disposed of and the new bus has been delivered to the district. Applications for replacement allowance will be filed for the fiscal year in which the replacement has been completed, and if allowed, reimbursement will be made during the next fiscal year. If the replaced bus remains in the possession of the district for a portion of the fiscal year following the delivery of the new bus, such replaced bus to be eligible for replacement allowance must be discontinued in use for pupil transportation purposes until disposed of or until converted for uses other than pupil transportation.
Repurchasing, renting, or leasing the vehicle back from the purchaser will void all reimbursement allowances granted under Sections 15323 and 15324 of this code.
(b) The Superintendent of Public Instruction may approve for reimbursement purposes the replacement of school buses on a two-for-one basis if he determines that the efficiency of operation of the transportation system will be increased, or that the operational costs will be decreased by such replacement. The approval shall be subsequent to and justified by a review of the district's transportation system including but not limited to fleet size, routes, pupils transported and related current expense of operation. In no instance will the combined reimbursement exceed the approved costs of the new vehicle(s).