Only the person that is legally obligated to pay the interest can deduct the interest.
Student loans can get pretty creative so if you were somehow listed as a co-signor or some other way obligated to pay, then you can deduct the interest.
If this is a true parent-plus loan where just your parents agreed to pay, then you cannot deduct it as you are not legally obligated to pay it.
All of the following must be true of the loan for the interest to be considered deductible: