Very similar experience as JP above - worked as a retail assistant for 3 months, but me being such an underweight and weak kid, I got really tired after work, as similarly dreaded the days I had to come in.
It really affected my grades, and as my parents said and JP too -
would you rather have that wage for a couple of years, or actually put the 27k tuition fee to good use and come out with an amazing class-degree? At the end of the day, it is advised for students not to work alongside studies, unless it is something very undemanding, like working behind a counter, or something that doesn't require much effort.
The on-campus jobs are the best, as you can work around study time, but it seems like you'll need these weekends in order to catch up - there's not point in trying to come out with no loan;
the loan is there in the first place to help you do your studies! Only then can you use that education to get yourself a good salary and pay it off - and even then in minuscule amounts.
I think quitting this current job would be best - at least now you know how demanding work can be. If you still need the income, then try and search for
something more simple, like working in the library for instance. I worked for 3 evenings a week and sacrificing time after-school didn't help with homework, nor revision. It made me tired, wasted away my motivation and was only for some money that now just sits in my account.
Hope you manage to pick up your grades soon - this year may not count as much, but it's only a practice run for the next two - and you can't afford to make any mistakes then