Including discount codes in membership imports
I am not sure why you need to import a discount code. A discount code is needed for online transactions so that civi knows to charge a reduced price yet still create a membership. Here the transaction has already taken place offline. 

Import the contributions for the actual amount paid, with a contributions source of, say, Student (or student discounted price if you think you might forget the student price was discounted).  Then import the memberships. There is no link between contributions and memberships when they are both imported, so the membership "won't care" how much money was paid for it.