Is a student sinful if he does not repay an interest-bearing loan he obtained from the state, knowing that it is the state's custom not to demand repayment of these loans from students?

Borrowing with interest is forbidden by Islamic law. The borrower is only required to repay the principal amount, without the interest, as Allah Almighty says: "And if you repent, you may have your principal – [thus] you do not wrong, nor are you wronged." If the lending party forgives the principal of the loan, then it is not obligatory to repay it. Otherwise, it must be repaid to clear one's conscience.