A member of the public employees retirement system who on the effective date of this section is employed as a fireman in a position requiring satisfactory completion of a fire fighter training course approved under section 3303.07 of the Revised Code or conducted under section 3737.33 of the Revised Code may elect to remain a contributing member of the retirement system by giving notice to the system not later than ninety days after the effective date of this section. The election once made is irrevocable.