Inspired by:

https://www.deviantart.com/arias87/art/Write-a-Short-GTS-Story-Contest-5-MHA-812426861 (also the source of the cover art)

Shrunk in MHA! (which I've written some chapters for)

The Dawn of Quirks resulted in the rise of superpowers in humanity in every universe where it took place. In this universe, however, it had another, equally significant effect: it resulted in the miniaturization of the male sex, resulting in a world where 80% of the total population has superpowers but 100% of the male population stands at an average of one inch tall.

Even in such a radically different universe, however, the dream of heroism never died.

40 new future heroes are about to start attending UA High School, the most celebrated hero academy in Japan, where they'll learn how to protect and serve as pro heroes in this world of giantesses. This is the story of their tiny hero or giant heroine academia-depending on which chromosomes they happened to be born with.

Setting Notes & Worldbuilding

• The heights for the male characters all follow the same formula. The average human male has been reduced from 175cm/68.898 inches tall to 1 inch tall. Therefore, when writing about a male character from canon, his height here is his canon height in inches (based on existing MHA height charts) x 1/68.898. This also applies to the linear dimensions of all male infrastructure or items.
• In accordance with the square-cube law, the weights for the male characters, and for male infrastructure, are 1/327054.286567th of what they'd be in canon.
• All the male characters' Quirks still work, but those where their effectiveness is affected by the size of the character in question will be proportionately scaled down to fit the setting. For example, Bakugo's Explosion quirk is based on him sweating nitroglycerin, and how much of it he can sweat determines how big his explosions are. So while he can destroy entire buildings in canon at his absolute strongest, here the worst he can do is blow up a (full sized) woman, and most of the time he's going to be way weaker than that. Quirks such as One for All, All for One, Erasure, and Half-Hot Half-Cold, however, are still just as inherently powerful as in canon, as they are not affected by the physical sizes of the characters. If you are confused about how strong a particular male character should be in this universe, feel free to send a PM.
• The size difference was caused by the same genetic changes that led to Quirks existing, and therefore also applies to trans women. If Magne is ever introduced to the story, she will be at the same height as male characters.
• Male infrastructure is largely built into normal-sized (female) infrastructure. For example, the walls of normal-sized buildings have grooves that serve as pathways for males or are equipped with trams, and normal-sized public transport has accommodations for male usage. In unisex bathrooms, male urinals and toilets are often built into female urinals and toilets, which also allows males and females to use the same toilet at the same time (though there are sight barriers in place to reduce embarrassment). There are also areas designated as male enclaves within otherwise normal cities (women can visit but only with permission), which are identical in function and looks to a whole city, except 1/327054.286567th in size. These design changes are also reflected in the changed designs for U.A's training grounds, the USJ, and the Entrance Exam site.
• The hero system here is different from canon MHA due to the need to to accommodate for the size difference between heroes and heroines. While supporting roles are carried out by either gender, it's the female pros that are much more visible on average as they're the ones who patrol the normal-sized streets or in normal-sized buildings. Male pro heroes, in contrast, work in roles that suit their much smaller sizes, either fighting male villains in places where pro heroines are too big to get into or helping rescue operations by searching in rubble and other tight spots. Exceptions are made for those few male heroes like All Might or Endeavor that have the power and/or skill to match even the strongest heroines. 
• If you have any other questions about worldbuilding and what might be different from canon due to all males having been tiny for a long time, feel free to get in touch with me.