Generally, Aizen loses to Ichigo and Urahara, but Urahara offers him a deal, Aizen has to go back to school. And Urahara and Ichigo don't kill him. Aizen agreed and was now sitting at the third table in his school uniform. Even though his classmates are not so simple and his class is bad, Aizen's class teacher tells him that he and his class will have a new student (that's you). Now when you enter your classroom, the whole class, including Aizen, is sitting at their desks looking at you