For retirement effective for May only, any member who completes all service for a TRS covered employer by the 15th of June, terminates all employment in public education in Texas by the 15th of June, and satisfies all other requirements for retirement before the 31st of May shall be eligible for retirement on the last day of May. In other circumstances, the retirement date shall be the last day of the month in which the member is last employed and satisfies all conditions for retirement. No member who has accumulated 12 months of service in the final year before retirement may use this rule to add compensation for any additional period of service to his annual compensation.