AS hard as working and doing good in school is, i think that its really important to be working, It helps with our transition from the high school setting to real life go much smoother. I don't think that working should be an issue, i manage 20 hours a week working plus I'm a competitive gymnasts which takes up another 20 hours a week and i still have really great marks in school. I think the key thing is to utilize the time given to work on assignments or for studying. Poor marks are often only seen in student that honestly don't care or aren't trying.