During the first test that Class 1-A students take after the Entrance Exam, Deku must throw a ball as far as he can. While doing so, his quirk doesn’t come out. Why did Aizawa erase Deku’s quirk?
Aizawa chose to erase Deku’s quirk because the student can’t control it. After each use, Deku is incapacitated and needs someone to help him. He won’t let him use his quirk when it means he’ll need help afterwards.
The Sensei then lets Deku have his quirk back and attempt the throwing test again.
Aizawa decides to erase Deku’s quirk during a test
Chapter #5 begins with a test of each students’ quirk. Aizawa, the Class 1-A homeroom teacher, wants to assess each student’s ability on the first day of class. Deku is able to pass the entrance exam, albeit barely.
Aizawa says that using your quirk during a gym-related activity is against the rules in the education system. Softball throwing, the standard long jump, the 50-meter dash, and other gym tests that students take in middle school, do not allow quirks.
Bakugo is then asked how far he could throw a ball in middle school. He replies 67 meters, and Aizawa then asks him to try again while using his quirk.
With the power of his quirk, Bakugo is able to launch the ball an impressive 705.2 meters.
Deku begins to panic. He realizes that having no control over his quirk is a recipe for disaster when it comes to this test. Aizawa then informs the class that the student with the lowest score across all 8 upcoming gym tests will be expelled from the school.
Next chapter’s event is the 50 meter dash. Those who can enhance their own speed, like Iida, Ochaco, and Tsyu, fare well in the event. Deku only receives a time that’s a few fractures of a second faster than his original time in middle school.
Next, the grip strength test. Despite remembering advice from All Might on how to acclimate his quirk for use, Deku still falters during this test. He prepares for the ball-throw, the last event that he may have a possible chance of not being ranked the lowest on.
Aizawa explains his reasoning to Deku
As Deku revs up his quirk to throw the ball, it only trickles out of his hand at measly 46 meters. “I erased your quirk,” Aizawa says over Deku’s shoulder.
Aizawa says the “ridiculous” entrance exam must be completely irrational to let someone like Deku in. He says he saw in the exam how Deku can’t control his quirk, and using it would just incapacitate the student again. He asks Deku if his plan was for someone to step in and help him afterwards. After each punch, Deku breaks his bones.
Deku’s plan to use his quirk would have inconvenienced everyone around him, Aizawa says. He also says that Deku has the same reckless streak as All Might, but becomes totally useless after saving just one person.
Aizawa then gives Deku his quirk back to try again.
So that’s the answer. Aizawa decides to erase Deku’s quirk because he can’t control it. Deku incapacitates himself after each use, and it would inconvenience his classmates to be knocked out. He lets Deku try again, but with a new mindset.