Can Atticus, Tom Robinson, Bob Ewell and Dill be considered secondary characters in the novel To Kill a Mockingbird?
All of the characters you have named play pivotal roles in this story, yet none is the central protagonist. Scout, whose real name is Jean Louise Finch, is both our narrator and our main character. And while Atticus's defense of Tom Robinson becomes a main concern of the novel's plot and climax, and eventually its resolution, Scout remains the eyes through which we as readers view the events of this story, and as such, she is the character with which readers empathize the most.
As for Bob Ewell, who could be considered our central antagonist, his role is important (as mentioned above), but he remains secondary as a character when contrasted with the radiant performances of Scout, Jem, and even Atticus. Due to Dill's limited presence and role in the novel itself, I would definitely relegate him to the label of secondary character as well. And Tom Robinson, while critical to plot development and evolution, is not our central player in this story, either. While your list contains many strong and identifiable characters, none is the protagonist, meaning that yes, they could be considered secondary, depending upon your teacher's definition.
The four characters you mention--Atticus Finch, Bob Ewell, Tom Robinson and Dill Harris--are actually among the major figures in the Harper Lee novel, To Kill a Mockingbird. Atticus is certainly one of the central characters in the story, and he gradually becomes the primary focus of the novel as the Robinson rape trial takes precedent in Part Two. The other characters can only be considered secondary to the primary character and narrator, Scout Finch, and, possibly to her brother Jem. Although Scout is the main figure, Atticus' powerfully drawn character eventually dominates the novel.
I wonder if your teacher wants you to look at the question in terms of growth and change in the novel. Although the story revolves around Atticus and his defense of Tom Robinson, those two characters, as well as Bob and Dill, remain constant throughout the book. The action is not only seen through Scout's eyes, the greatest change occurs in her; she is the one processing the events and gaining a greater understanding of human nature. So in a sense, yes, the other four are secondary to Scout--even though, as bullgatortail says, Atticus dominates the story itself.