I read this Korean manhwa about 2 years ago. I am bad at names, which is why I don't remember the title of the book, nor the names of any of the characters. This is not a popular series, and may not even have an anime. The manhwa is complete.
It has street fights. There are no superpowers whatsoever, as they are all humans. It's all about a guy becoming the top fighter in the school completely by accident.
I believe it takes place at an all boys high school, but there is a girl whom everyone mistakes for a guy attending the school for some reason. There are technically no rules against girls going to that school, though.
The main character is a complete wuss, with brown hair. In the first fight (maybe, not too sure), he pretends to be tough, then a group of people try to beat him up. However, he accidentally beats them all up instead. After that fight, I think pretty much all fights from that point on are 1 on 1, as people try to take him down and fail by sheer bad luck.
Even with all the fights going on, the parents are never involved, though I believe the principle is, and he literally beats up any kids that get into fights.
There's also a less popular remake where he can fight, but he keeps losing to everyone he tries to fight with. Basically, a complete flip of the series.
Feel free to make pot shot guesses, I will look up all answers.