As a child, Sanji always received comments on how he looked like a girl. He usually shrugged these off, becoming only mildly affronted when they started to insinuate that due to his girly appearance he was weaker than others. Luckily he knew that he was stronger than most of them, no matter how feminine he appeared, this line of thinking disbursed any fights before they could pop up.
Well, until now, anyway.

"Girls can't fight, especially girly boys like you! You're a pansy!" As soon as this green-haired boy had walked up to him with his utterly confused face Sanji had let out a sigh, already knowing what he was in for. No doubt the other had taken in his appearance, written him off as some tom-boy and left it at that, only to be surprised that he had joined the males in PE. Now he was here to play twenty questions with the blonde who just wanted to finish his lunch in peace. The trials of joining a new school, having to wait until every one simply got over how he looked and left it alone.
Sadly, though, after he had confirmed that yes, he was indeed a boy, Sanji's in-built snark reared its ugly head in the form of the words,

"and I'd like it if you left me alone, Marimo". Turns out the other boy hadn't the faintest clue what a marimo was and took great offence because of that fact, thinking it was an insult. Although the fact that he was not entirely wrong did not ease the sting of the other's following words, nor Sanji's ire that quickly grew to a boiling point after them.

"I bet you're so girly you don't even know how to throw a punch! huh, pansy?" That moss topped idiot had officially crossed the line, to Sanji there was only one way to deal with those who dared set foot over it. Prove them absolutely wrong.

In Sanji's mind, flashed a thought blaring like a large 'note-to-self'.
'He wants to know if I can punch? Well, it is said that if you stand up to the toughest then all the others will leave you be.' He had only just enrolled in this school and already he was being given lip over his looks, so perhaps this would help in that regard?

So, with all the ten years worth of strength Sanji had at his current disposal, he reared back his clenched hand, savouring the shocked look that slowly overcame the other's face when he realised what Sanji was about to do, then let his fist fly as fast as he could propel it into the annoying prick's nose.

The office had called Zeff, Sanji's guardian, in order to inform him of what had transpired between his charge and the other kid, Zoro as Sanji had later found out. He was asked to come collect Sanji as soon as possible, immediately in other words.

As Sanji sat next to Zeff, Zoro and his own guardian a little ways off to the side, he held his head high. He refused to feel at all sorry for what he had done, the other had challenged him, he had simply acquiesced.

"Sanji, at the very least, you need to apologise to Zoro and his father." The principal's tone was exasperated, his face had lost the surprised look it was set in earlier. Turns out this 'Zoro' kid got into a lot of fights and almost always was the instigator, so seeing someone else being the one to start the physical aspect came as a nasty little shock to him.
"If you do it, we'll take a couple days off of your suspension." This was said after Sanji shook his head to the initial order, the principal's offer was only met by the same reaction.

A pleading look from the school's leader to Zeff proved futile, Zeff had his head held high, similarly to his charge. No help was coming from him any time soon, he had taught Sanji to look out for himself, if he had judged that this violent action was required then who was he to contradict himself?

"But he proved me wrong, he doesn't hafta say sorry." Even though his words were both muffled and twisted, by the ice pack pressed to his face and the possibly broken nose he was nursing, respectively, Zoro's words were still audible.
Gingerly, Zoro slid off his seat, all eyes on him, and made his way over to his blonde assailant.
"I'm Zoro, sorry for sayin' ya couldn't be strong withou' bein' a propa boy." He extended his hand, the one not currently attempting to ease his pain, towards Sanji, who was staring slack jawed at him. "I think ya pretti strong!"

The minute Zoro's lips had twisted into that confident smirk, Sanji found his own lopsided smile slipping onto his face in reply. Looking back on that hand shake between them, the two would say it was what truly sealed their violently beautiful friendship.

Ughsos: 'In elementary school I hit this kid 'cause he said I can't punch and I broke his nose and then my step-Dad picked me up and the office was like "you have to say sorry" but then the kid was like "but she proved me wrong, she doesn't have to say sorry".
Ladies and gentlemen, my best friend of many many years.'

(I don't know, it sounded a lot like how I'd imagine Sanji and Zoro meeting as kids. This is either before Zoro met Kuina or she didn't exist for him in the universe.)