tell the GF that you need to focus on studies and spend tmie with her on the weekends. if she questions if you are not important say the following:
tell her that your classwork has to be done or you are out - they don't love and care for your or can compromise
tell her that work is what you are doing to get paid and survive - they won't love you and compromise to pay you more at less hours per week
tell her that she is the only one you love and care - and only she can understand how this is draining you, and compromise with you...because only she loves you
she can't say no to that

(and for those who ask - yes i just totally made this up on the spot)