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Here's some explanation:

http://www.calstrs.com/Legislation/Federal%20Legislation/ssandbenes.aspx

Social Security & CalSTRS Benefits

CalSTRS members do not contribute to Social Security (although members hired on or after April 1, 1986, and many members hired before that date contribute toward Medicare). As a result, the retirement benefits you receive from your school employment are provided entirely by CalSTRS. Nonetheless, there are two important issues concerning Social Security of which CalSTRS members need to be aware. First, discussions in Washington D.C. about efforts to reform the Social Security system to provide long-term fiscal stability to that program have included mandating that all state and local public employees hired in the future, including public school teachers, participate in Social Security. Second, although CalSTRS members do not receive Social Security benefits by virtue of their public school employment, many receive such benefits from other employment, or by being married to others who were employed in jobs covered by Social Security. A a result of federal law, the Social Security benefits that a CalSTRS member receives is often reduced because the member is receiving a retirement benefit from CalSTRS.