Being different means being bullied - moderately, in the beginning, because Sherlock dresses in sensible, practical, comfortable clothes and he looks rich. It gets a little bit worse when he starts pointing out people's flaws, the little secrets they haven't told their friends about, given away in details that are obvious to Sherlock but not, apparently, to anyone else.
Sherlock learns how to fight - from books and thus poorly, at first, but he gets better after he meets some friendly old man after school whom he helps carrying a bag of groceries and who offers to teach him in return for his kindness.
(He's entirely aware that this is Mycroft's doing, that the old man probably would have offered to teach him even if Sherlock hadn't helped him, but one needs to be practical in these matters.)
(Plus, he knows Mycroft will be entirely too smug about having foisted the lessons on Sherlock to even consider letting Sherlock know about the true state of things, which in turn means he will never call this in as something Sherlock will owe him for later.)
Mummy doesn't like it when Sherlock gets into fights, which is manageable; Sherlock acquires some make-up and hides away some spare clothes in carefully selected locations, and those times when Mycroft doesn't spoil things for him, he pulls it off perfectly.
It is, however, a lot trickier to manage Mycroft.
One would think he'd be pleased at Sherlock putting his lessons in martial arts to use, but all Mycroft seems to do is tut-tut and then talk to people who talk to other people who come and beat up whomever was trying to beat up Sherlock the next day, assuming they're still walking. (They generally are, because Sherlock has been taught to be compassionate and also not to get himself a police record.)
"I could tell Mummy about this," Sherlock says. Jerry McLane's face hits the pavement with a sound that does not bode well for his chances of impressing any girls with how handsome he is in the coming weeks. "I don't think she'd like it that you did this."
Mycroft turns and starts walking away. Sherlock catches up in two steps - sooner or later, someone will come and see what the noise is all about and it will be better not to be around by then.
"I believe you will find I have plausible deniability," Mycroft says, smiling even more broadly when Sherlock frowns at him (it's not his fault the librarians won't let him read anything for adults). "You can't prove it, and I will deny everything."
It annoys Sherlock when Mycroft is right. "If I'd recorded this conversation, that would be proof."
"Of a kind. But you didn't," Mycroft says, looking smug.
(Sherlock gets a small recording device that's disguised as a book for his next birthday.)
Mycroft collects minions, and Sherlock collects facts, because he's leanred that when you put facts together the right way, you get even more facts, and when you've got enough facts, you can find out about anything.
It makes the world a very exciting place for all of two weeks, until Sherlock finds out about his dad, who's supposed to be living on Mount Everest or possibly in the Himalayas.
When Sherlock gets the flu, Mummy takes care of him, feeding him, bathing him ... making him drink a medicine that tastes so utterly disgusting it can't possibly be good for anything, not really, except that she insists it does, and Sherlock is too physically weak to make his refusal to swallow it stick.
Mycroft isn't allowed to come visit him, but he smuggles in books on chemistry and biology and how the human body works (terribly inefficiently, Sherlock concludes in disgust) under the cover of 'homework assignments' which Sherlock initially insists he isn't nearly well enough for. (It's not that he is, really; it's that he feels it would be only fair if he wouldn't need to bother with boring, tedious homework when he's already got a headache from the flu.)
"This still doesn't explain why that medicine tasted so bad," Sherlock tells Mycroft, after, when he's returning most of the books. (He's kept the one on poisons, including the note about why flu medicine is nothing whatsoever like poison and could Sherlock please not upset Mummy again by suggesting that she might have switched two bottles or some such thing).
Mycroft inspects his books like he expects to find ripped out pages. "Some things simply are."
(This is how Sherlock learns that, sometimes, Mycroft can be a fool.)