To what extent must one be selvish

My first post will be about my job and my experience at work, so let's begin.
As a part time employee I wouldn't expect much from my job.
Not the best pay, not many work hours, not many responsibilities, etc.
I mean, I'm just frying food and putting it together, you know? I didnt thought it would be bad.
However, this past 2 months I've felt chained to my work for reason that conflict with what's best for me.
I'm a high school student and by no means I want to put work over school, yet I have to do exceptions every now and then because of my personal life or others mischief.
Which I understand, things happen, but I'm not sure if am I being unfair with my coworkers by deciding to work less hours (I don't want to let them down) or to disappoint my teachers and myself who expect better than how I'm doing at school.
To what extent must one be selfish? This is the question that I then come in conflict with because I'm just tired and exhausted from my life (I'm in no way suicidal btw).