If I used my fathers credit to buy myself a tablet for school, can I still put that as an expense for school even though I haven't finished paying it off? Or would I only be able to put down what I have payed off so far?

Even if you used a credit card (or account) to pay for the computer, you still bought it and can claim the full value as an education expense, if otherwise qualified. 

The same applies to qualified education expenses you paid for with student loans.