What is the ruling on seeing a female student cheating on an exam or cursing a teacher without forbidding her from doing evil, and is it possible to repent from that?

Deception is an abominable and forbidden act whose harm is not limited to the one who commits it. The Prophet, peace and blessings be upon him, said: "Whoever deceives is not of us." Therefore, the deceiver must be advised to abandon deception and have its ruling explained to him with wisdom. This falls under enjoining good and forbidding evil. The same applies to one who curses or insults, for the Messenger of Allah, peace and blessings be upon him, said: "A believer is not one who slanders, or curses, or is obscene or vulgar." And whoever has fallen short in the duty of advising and enjoining good and forbidding evil must repent to Allah and resolve to fulfill this duty for the remainder of his life.