In literature, an inciting incident is one that effectively begins the story. Prior to the inciting incident, everything that happens is pretty much backstory, but afterwards, the story proper gets under way. Now, there is a problem to be solved, and suddenly the protagonist is thrust into the main action of the story. This is precisely what happens to Atticus Finch in To Kill a Mockingbird. The alleged attack on Mayella Ewell will change his life completely. Almost everything that happens from here on in is related to the incident. Atticus has been presented with a problem, one that he must somehow attempt to solve. It's a tall order, to put it mildly. This is a time and a place where an accusation of rape by a black man against a white woman is tantamount to a death sentence.

In terms of the plot, the inciting incident reveals so much to Scout about herself, her family, the townsfolk of Maycomb, and human nature in general. It's not just Atticus, then, who must try and solve the problem created by the inciting incident; Scout must also figure out how to deal with the numerous revelations that arise as a result of Mayella Ewell's alleged assault. This incident will change Scout's life forever; it forces her to grow up fast. The problem established by the Mayella Ewell incident is inextricably linked, then, with the normal growing pains of a young girl.

Your teacher is correct in saying that the inciding incident is the violent scene that is only described after the fact by witnesses at Tom Robinson's trial. Mayella, the young daughter of Bob Ewell, according to Tom's testimony, asked him to come inside the house, where she made vigorous sexual overtures to him. He tried desperately to escape, because he knew that it would almost certainly be fatal for a black man to get involved with a white woman in that period of Southern history. Unfortunately, Ewell saw through the window what was going on inside. He beat Mayella with his fists, then charged Tom with attempted rape and made his daughter promise to tell the same story. Atticus demonstrated in court that it would have been impossible for Tom to inflict the bruises because of his withered arm. It comes out in the testimonies that Ewell must have had an ongoing incestuous relationship with his daughter.