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Someday I'm going to pull that mask off of him for everyone to see!!
Goodbye Mr. Smith (さよならエルヴィンSayonara Eruvin?) is the 4th chapter of the 4th volume and the 27th chapter overall of the Attack on Titan: Junior High manga, written and illustrated by Saki Nakagawa.
Nile recalls a field trip he went on with Mr. Smith and several first year students a few years ago. He rages that everyone favorably views Smith as a teacher despite his dislike of Titans, only to hear that Smith has claimed responsibility for attacking the Titan students and plans to resign.
Summary
Nile walks down the hallway in school and greets two students that pass by; he overhears them compare him negatively to fellow teacher Mr. Smith and goes to the roof of the school to vent his frustration. He mentions how Smith pleases everyone and bitterly wonders why no one can see his true nature.
He reminisces about an event he witnessed two years earlier; Nile took part in a filed trip with Erwin and several first year students. One of them, Hange, cries over how her lunch was stolen but Mr. Smith volunteers to help find some edible food on the mountain they were at. Nile scoffs at this and goes to have his lunch in private.
He stumbles upon a group of Titans that are surrounding a pile of lunches taken from students; he turns around and spots Erwin right behind him. Erwin asks if they should confront the students, but Nile sees it as a futile gesture and disagrees. Erwin then takes it upon himself and beats the Titans; when Nile tries to tell the other instructors, they do not believe him.
As Nile continues to rage against Mr. Smith, another teacher finds him and informs Nile that Smith admitted to being the one who attacked the Titan students and will be resigning his position. As several of the students try to defend Mr. Smith, he affirms that it was true. Nile arrives and starts to brag in Erwin's face that he was right, mentioning the rules will stay in place. After Eren and several other students arrive, Erwin hands his resignation letter in only for Eren to grab it and tear it to pieces.