SECTION 1: Subsection (1) of section 4 of chapter 32 of the General Laws, as appearing in the 2022 Official Edition, is hereby amended by inserting after paragraph (g) the following paragraph: (g1/4) A public school teacher who is a member of the teachers’ retirement system, a regional retirement system, or the Boston retirement system, and who has completed at least 20 years of service may be credited with creditable service for part-time employment as a teacher, upon their return to full-time service, for the purpose of determining their superannuation retirement allowance, on a proportionate basis which the board shall determine according to rules and regulations adopted by the board and approved by the commission. The board shall credit as full-time service that period of time during which a member performed part-time service as a public school teacher, not to exceed 5 years. Eligibility for creditable service under this subsection shall be conditioned upon payment by the member to the retirement system, in an amount equal to that which would have been withheld as regular deductions from their regular compensation for full time employment, in 1 lump sum or installments, as the board shall proscribe, together with regular interest.