From dear old wikipedia
Groups not covered by Social Security
There are a number of groups of workers who are exempted from having to pay Social Security taxes:
Federal employees hired before 1984 who elected to continue to participate in the federal retirement program instead of receiving part of their retirement under Social Security coverage.
State or local government workers (police officers, firefighters, and teachers) hired before March 31, 1986 and participating in their employers' alternative retirement system.
Ministers may choose whether or not they will participate in the Social Security program.
Self-employed workers with annual net earnings below $400.
Election workers earning $1,000 or less a year.
Household workers earning less than $1,500 per year.
Minor children with earnings from household work but for whom household work is not their principal occupation.
College students working under Federal Work Study programs, graduate students receiving stipends while working as teaching assistants, research assistants, or on fellowships, and most postdoctoral researchers.
Individuals who are members of certain religious groups such as the Amish and Mennonites.
Before the 1983 changes, three counties in Texas (Galveston, Brazoria, and Matagorda) opted out of the system and now use an Alternate Plan, a private pension plan created and administered by First Financial Benefits, Inc.
In 1983, the U.S. Congress closed a loophole in the original Social Security Act that allowed municipal governments to opt out of the Social Security system, and also brought all civilian federal employees whose employment began in 1984 or later under the system.