Do I need to pay social security and medicare taxes if I work part time as a graduate student assistant in my school while I'm also a full time student?

My W-2 does not have any medicare and social security tax withholding. For the past 2 years I was in a similar situation and also filed a 1040A for federal taxes as a dependent on my parents. I earn less than $5000 a year from my graduate school.

Recently I received a letter from social security stating that I did not pay any social security taxes. This is correct since my school did not withhold them from my paycheck for 3 years and I've only filed two 1040A for federal taxes only so far.

What should I do in this case? My W-2 this year also did not have any social security taxes. Do I need to request my school to withhold it or do I just not need to pay social security taxes?

Please advise. Thank you.