It sounds as if they may have you confused with someone else. Call the financial aid department and stay on the phone until you can talk to a real person.
If you are going to be getting Stafford Loans you can choose any lender on the list. You do not need to stick with Sallie Mae. Also, if your current loan is a Stafford Loan and you will be in school at least half time you can request a deferment from payments while you are taking classes.