Social Security in OPT??

I was in OPT (practical training) last year. I worked for a company and paid Social Security, Tax, and Medicare for the whole OPT period. One of my friend told me that I do not have to pay Social Security and Federal Tax during OPT. Is it true? If it is true, how can I get the money back? Any special documents or paperwork to prepare? I plan on leaving U.S. in two years, so there is no point to pay Social Security for my retirement.

I appreciate your help