When you turn 65 you can sign up for Medicare Part A. There is no charge for this. It covers hospitalization. If you have TRS-ActiveCare for your medical insurance, it remains your primary insurance as long as you work for the district -- Medicare Part A is your secondary. Medicare Part B (approximately $100 per month) can be DEFERRED until you no longer have Group Health insurance through the district.
See www.medicare.gov for more information.