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Right after Shin and the others entered the auditorium-type lecture hall, they immediately looked for their seats.
Shin immediately went to the highest point, where the back-row seats were located. Unlike a regular studious student, he always chose to sit in the back row, farthest from the teacher's stand because of Arthur's insistence.
After studying being with such a guy, he had gotten used to it, and became a habit of his. For him, it would be fine wherever he sat when attending class. What was important was whether he was paying attention to the teacher or not.
As for Arthur, that guy obviously chose to seat that the back row to sleep in class. Though, with the sitting arrangement within the lecture hall, where the furthest row was on an elevated position, the professor would immediately spot him if he tried to sleep in the class.