This might be useful to you:

"A nonresident alien temporarily in the United States as a nonimmigrant on an "F-1," "J-1," "M-1," "Q-1," or "Q-2" visa is not subject to social security and Medicare taxes on pay for services performed to carry out the purpose for which the alien was admitted to the United States. Social security and Medicare taxes should not be withheld or paid on this amount. This exemption from social security and Medicare taxes also applies to employment performed under Curricular Practical Training and Optional Practical Training, on or off campus, by foreign students in "F-1," "J-1," "M-1" or "Q" nonimmigrant status as long as the employment is authorized by the U.S. Citizenship and Immigration Service."

http://www.irs.gov/businesses/small/international/article/0,,id=106287,00.html