What is the legitimacy of a relationship between a high school student and a teacher twelve years his senior, whom he considers like an older sister, and with whom he speaks on the phone and meets during holidays, with the knowledge of her father and fiancé?

Allah has forbidden looking at and mixing with foreign women except out of necessity, such as for teaching or medical treatment, as a preventative measure against temptation. This is based on His exalted saying: "Say to the believing men that they cast down their looks and guard their private parts; that is purer for them; surely Allah is aware of what they do" (24:30), and His glorious saying: "And when you ask them for anything, ask them from behind a curtain; that is purer for your hearts and their hearts" (33:53).

Moreover, the Prophet, peace and blessings be upon him, clarified the danger of the gaze, saying: "Indeed, the gaze is a poisoned arrow from the arrows of Iblis." He also explained that the eyes have their share of adultery, which is looking; the ears have their share of adultery, which is listening; the tongue has its share of adultery, which is speaking; the hand has its share of adultery, which is striking; and the foot has its share of adultery, which is stepping. The heart desires and wishes, and the private parts either confirm or deny that.

Based on these evidences, the mentioned relationship is a sin that necessitates immediate repentance, by severing the relationship, regretting what has passed, and resolving not to return to the sin. And Allah accepts the repentance of those who repent, as stated in His exalted saying: "Say: O My servants who have transgressed against their own souls, despair not of the mercy of Allah; surely Allah forgives all sins. Indeed, He is the Forgiving, the Merciful" (39:53), and His saying: "And He it is Who accepts repentance from His servants and pardons the evil deeds and knows what you do" (42:25).