What payroll told you was wrong. They took out social security and medicare from your paychecks without taking in consideration any tax treaty benefits. International students, nonresident aliens for tax purposes, are not subject to Social Security and Medicare taxes also known as FICA taxes.

Here is what you can do:

If you are an international student on Optional Practical Training - OPT, nonresident alien for tax purposes, and your employer has deducted social security and medicare taxes in error, you must first ask your employer for a refund. If you are unable to obtain a refund from your employer, you must file Form 843 and Form 8316 to obtain a refund from the IRS for the Social Security and Medicare taxes withheld in error by your employer.