What should I do to atone for mocking and ridiculing some teachers, given the difficulty of confessing to them or supplicating for them, and is a general supplication sufficient, or must I specifically supplicate for each person I have backbitten?

Mocking people is forbidden, as Allah Almighty says: "O you who have believed, let not a people ridicule [another] people; perhaps they may be better than them; nor let women ridicule [other] women; perhaps they may be better than them." It is a form of arrogance, and no one who has an atom's weight of arrogance in their heart will enter Paradise. Remorse is a sign of accepted repentance. The completion of repentance is achieved by abandoning the sin, regretting it, and resolving not to return to it, along with frequent seeking of forgiveness and good deeds. As for those who were slandered, one should generally seek their pardon and forgiveness without detailing the specifics, unless detailing would cause hatred and resentment. In that case, it suffices to pray for them in their absence and mention their good qualities.