Bloody knuckles with bruises just starting to heal was probably one factor of why the nickname started. The other reason most likely being the fact that a young, fresh blood student was fighting one of the upper classmates. A big guy too, twice the height and muscle mass of the sixteen year old. There were mixed shouts.
Some yelled for the stubborn boy to give up, others shouted him on...well, they just wanted to watch a fight. They didn't care who won, the thrill of watching was their drive for shouts.

But Sebastian tuned them out. Even at the innocent (well, innocent compared to his adult life) age of sixteen, he knew how to evaluate a fight. To see weak points, good places to hit...everything he knows today rooted from the fighting teen spirit.
And of course, the teen could see that he was most likely about to get his arse handed to him, ignoring the slim chance of winning if he played his punches right.

While this high school brawl behind the school was far from the horrific fights of the future, it certainly prepared him. Taking on fellows bigger and stronger than him, and how to survive in those situations. And this strategy was dumb but...effective, which would become a rare chance in the soon to be sniper's life.
Wait till the opponent gets tired, stay light on your feet. Dodge the hits, then go for the kill.

And that's what he did, a risky waltz of his own. The fists trying to pound his stomach or face were his tune to dance to, dust rising from his skidding feet like the skirts of a twirling dress. It was a rush alright, his heart a drumbeat in his ears. Just keep dodging, wait for the time...then pounce, little tiger. He kept his eyes locked on the taller teenager dancing with him.

Then his time came. As quick as a drizzle of rain, Sebastian delivered flurries of punches to varying places, sometimes earning an extra minute to get another in after making his tough opponent bend over in pain. This lasted a while, to the point that Sebastian's muscles got sore, he was burning in his clothes. But he kept going on.
Dodge.
Wait.
Attack.
Wait again.
Attack again.

And then a cry rose up as the odds went from "loser" to "winner". The person who started it was well known; a classmate of Sebastian's, who was a victim of bullying...and the Robin Hood esque teenager fought off everyone who bullied her, and any attempts. This fight including an attempt.

"GO, BASHER, GO!"