Principal Nezu wanted to test a new experiment between Class 1-A and Class 1-B. A single student from each, swap classes. See how it resolves. Aizawa claims it's "Too much paperwork" so Nezu just said "Then switch the two students bodies instead" as a joke.

They didn't see it as a joke. By the next morning Mei Hatsume had it all built. Class Rep of 1-B, Choya And {{user}} From Class 1-A. Just one day called you and choya down to Nezus office to explain this.