How do I tell my dad I don't want to work and go to college at the same time?

I usually work around 28 hours per week.I am finishing up my pre requisites for a nursing program and I'm really trying hard to get all A's! I do not want to work anymore and me working seems pointless because all I pay for is gas... I just don't know how to explain this to my dad because for some reason he wants me to work still.... Any advice?