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.