Opportunity cost is the value of the thing or things that you had to forego in order to do a certain thing. In this case, the opportunity cost of your job is the value of the things that you will not do while you are working. Your opportunity cost will depend to some degree on your personal circumstances and what you like to do. It might be grades; you might see your grades drop as you have less time to study. It might be social opportunities. You might have less time to hang out with your friends. Your opportunity cost is simply the value of all the other things you will not be able to do because you have this job.