The other teacher is correct. In chapter 30, Heck Tate and Atticus are discussing what happened and Atticus thinks at first that Jem was the one that killed Bob Ewell. Heck insists on calling the death an accident, but Atticus, ever the fair and impartial lawyer, doesn’t want Jem protected from the law. Heck insists that Ewell fell on his knife and that Jem didn’t kill him. Heck knows that Boo is the one who stabbed Ewell but he wants to keep the facts secret. He says that Boo, with his "quiet ways", doesn’t need the entire town bothering him any more than they have in the past. He reminds Atticus that Tom Robinson died for no reason and now the man responsible for that miscarriage of justice (Bob Ewell) is dead. “Let the dead bury the dead,” he tells Atticus.
Some of my students have taken issue with this ending - the fact that Atticus agreed to keep things quiet. They said it was out of character - that a man who was even willing to allow his own son to be exposed to the workings of the law (when Atticus thought it was Jem who killed Ewell) would not have so easily agreed to hush up Boo's deed. What do you think about this? I think Atticus did act according to character because sometimes we must listen to a higher power, and surely that power would not have wanted Boo to suffer any further abuse.
I've taught this book many times. I don't have the novel in front of me right now, but I remember that Heck Tate said that Bob Ewell fell on his knife. I don't believe that he physically altered anything; he was just going to tell a different story based on the evidence that he saw. He refused to implicate Boo Radley because Boo is yet another "mockingbird" that should not be hurt.