What is the ruling of Islamic law regarding demanding money that was spent by a person who received it by mistake, knowing that he is still a student and cannot pay it back all at once, and will he be held accountable for it on the Day of Resurrection?

You should have informed the bank when the money arrived in your account, and you erred by agreeing to repay it monthly and then procrastinating on fulfilling that. You must now secure the full amount and repay it directly to its owner. If you are unable to do so, then keep it as a trust and write a will to that effect. If you cannot find him or his heirs, then donate the money on his behalf, informing him or his heirs later so they can choose between the charity or their money. Repentance and seeking forgiveness are obligatory. If the rightful owner refuses after your best efforts, it is hoped that Allah will bear the burden for you.