Am I sinful or blameworthy for my repeated failure in high school and my shortcomings? And what is the ruling on my father's action of taking the computer he gifted me, and is it permissible for me to reclaim it from my younger brother?

Neglecting studies or duties without a valid excuse is reprehensible both in Islamic law and in reason. The one who neglects should seek God's forgiveness and resume life in a manner that pleases his Lord and his father. Indeed, God loves excellence and perfection in work. The questioner must excel in his studies, seeking reward, benefiting his nation, pleasing his father, and avoiding his displeasure, for the Prophet, peace and blessings be upon him, said: "The pleasure of Allah is in the pleasure of the father."

Regarding the father taking back the computer he gifted to his son, there is no sin upon him for doing so, and the son is obliged to obey him. It is not permissible for the son to take the computer from his younger brother if his father had given it to him, because revoking a gift after its acceptance is not permissible except for a father regarding what he gives to his child, for the Prophet, peace and blessings be upon him, said: "It is not lawful for a man to give a gift and then take it back, except for a father regarding what he gives to his child."