It really depends on the credit union and how the loan was and will be assessed. Reading between the lines I'd hazard a guess that it was underwritten on the basis of your mother's repayment capacity rather than yours, and with a guarantor in addition to this you should be more than OK for such a small amount (assuming that the overall loan balance isn't massive). In any case, the CU wouldn't necessarily refuse because of an overdraft -there's often a bit of flexibility on student loans. You're best to apply and they should advise the best course of action to get it approved.