A regional occupational center established pursuant to Chapter 9 (commencing with Section 52300) of Division 4 of the Education Code by two or more school districts by a joint powers agreement shall be deemed a school district for purposes of this part. The board and the county superintendent of schools, upon the request of the governing body of any center in the county, shall amend the contract entered into under this chapter to include the employees of the center who are not eligible to membership in the State Teachers’ Retirement Plan. Credit shall not be granted for any service in that employment prior to the effective date of the amendment. However, on the request of the governing body of the center, the amendment may provide that the membership of any person becoming a member in that employment on the effective date of the amendment shall be retroactive to the date of that person’s entry into that employment. If the amendment provides for the retroactive membership, both the member and the center shall contribute to the retirement fund for the period the amounts they would have contributed had the amendment been in effect on the date of the entry into employment.