Social Security & Medicare taxes while on OPT?

[JLMA]

I am on OPT.
I have two jobs:
(i) for a state University: they withhold certain % for State taxes and Federal taxes
(ii) for a private company: they withhold for State & Federal taxes, and ALSO for Social Security and Medicare.
IS THIS CORRECT?
Could I have the Social Security & Medicare taxes be waived in the private sector job? After all, I am in the USA only for 5 more months.
Thank you.
JLMA