My Hero Academia's primary setting is the U.A. hero school, and getting into it is not easy. Each year, thousands of Japanese teenagers apply to get in and only a handful of them are accepted into the new class. Series protagonist Izuku Midoriya saw for himself how tough the entrance exams can be, and this begs the question: how did his homeroom teacher, Shota Aizawa, a U.A. alumnus, ever get in?
Nothing of Aizawa's student days are shown in the main series, but the Vigilantes spinoff clearly depicts Aizawa and his friends, Midnight and Present Mic, as students, and even as a second-year, Aizawa was struggling. His Erasure Quirk is poorly suited for passing a combat-oriented entrance exam such as the one Izuku and others faced, so perhaps Aizawa took an alternate route to get into the Heroics course. The online My Hero Academia community certainly thinks so.
Aizawa Might Have Transferred From The General Studies Course
Classes 1-A and 1-B are both in the Heroics department of U.A., but there are other departments too, such as the Support department, and the General Studies department. One fan theory states that since Hitoshi Shinso was moved from the Gen-ed department to the Heroics department, then surely Aizawa must have done something similar as a student. Shinso is Aizawa's protege and mirror, especially in terms of their Quirks, which are effective but cannot deal damage to enemies or target dummies.
Shinso stood no chance of passing the physical exam to get into the Heroics course, so he went to Gen-ed first, then proved himself against human opponents during the Joint Training battle between Classes 1-A and 1-B. Aizawa could have done something like that in his own day since his Erasure Quirk does nothing against practice dummies but is quite effective against human opponents. However, there is another, even more compelling possibility.
Aizawa Might Have Made It In Without A Physical Exam At All
Another fan theory uses evidence from the Vigilantes spinoff manga to make another extrapolation: that Aizawa didn't take part in the physical exam, nor did he end up in the General education department at any point. His friendship with Midnight and Present Mic in his second year is a vital clue. If he had switched departments early into his time at the school, then he wouldn't have been their well-established friend by that point. He must have gotten to know them during his first year, likely from the very start.
Another piece of evidence is that Aizawa already had his Provisional Hero License in the summer of his second year. It's unlikely that he could have obtained it if he had transferred departments partway through his U.A. career. If he had transferred, it's tough to believe that Aizawa would have gotten enough hero training to pass the Provisional Hero Iicense test, so he must have been in the heroics department since his first day at U.A. That's what it takes to pass the exam. It should be noted that Izuku's Class 1-A is the exception for passing the test as relatively inexperienced first years -- ordinarily, it's second-years and third-years with extensive training who take the Provisional Hero License exam that have a chance of passing it.
This fan theory also posits that Aizawa got placed directly into the heroics department simply on merit of his Quirk, which may have been seen as too precious to waste in the Gen-ed department. His Quirk cannot deal damage, but it is extremely rare, and U.A.'s staff must have recognized its potential right away. So, U.A.'s staff probably made a rare exception for Shota Aizawa and allowed him to join the Heroics department if he could pass his written test, which he did. It would be an unfair waste of such a remarkable Quirk to bar Aizawa from the Heroics department just because he can't damage test dummies in live-fire exercises.
The final piece of evidence is that the giant robot test is a new one. Just a few years ago, the theorist argues, the entrance exam was different. Perhaps that test involved actual human opponents, and if so, Aizawa wouldn't even need a special exemption from U.A.'s staff; he could simply use his Quirk as intended and defeat his human opponents in personal combat.
All of these clues make it clear that Aizawa's process of gaining admittance to U.A.'s Heroics course had been much different from Izuku's own, and for that matter, probably different from Hitoshi Shinso's, too. The two strongest possibilities as to how he got onto the Hero Course are either that Aizawa defeated human opponents in his physical exam, or he was allowed to get into the heroics department with a passing written exam score alone since his Quirk is too precious to waste in a physical exam against non-human targets. If so, U.A.'s leaders were wise to bend the rules and make this deception. Not all heroes need to level city blocks to be powerful.