J.K Rowling owns Harry Potter, not me (sadly.) This is set in Sirius's 3rd year. Sorry if some of the facts aren't quite right but it needed a little tweaking to work :)
Sirius Black was proud to say he was a true Gryffindor and nothing like a proper Black. But there are some things that a lifetime of rebellion can't rid.
It was Snivellus's fault, Sirius thought angrily as he paced the corridor outside McGonagall's classroom. Right now he wanted to do nothing more than pull out his wand and curse Snape until he screamed. But he couldn't do that because of the lesser reason that he would be expelled and the first and foremost thought in Sirius's mind: it was something a Black would do.
The door opened and the class poured out, James giving him a grin and Remus and Peter looking anxious as normal. And Snivellus glaring at him through his greasy mop, the coward. He'd get him back for this, just wait and see.
'Come in mister Black,' Sirius was jerked out of his daydreams as McGonagall followed her students out, holding the door open for her disgraced student. 'Sit.'
Feeling slightly irritated at being treated like a dog, the fact that he was one not withstanding, Sirius sat and proceeded to glare at a point a few centimetre's to the left of her hat point.
'Mister Black, can you tell me why, after mister Snape tried to aid you in your failed transfiguration, you saw fit to curse him?'
'He was sabotaging my work, the filthy Slytherin!' McGonagall's mouth thinned, a warning sign for all sane students to stop talking immediately. Unfortunately for him, Sirius was about as sane as his cousin, Bellatrix. Which was to say, not much.
'He was aiding another student mister Black. And just because he is a Slytherin does not give you an excuse.' Sirius almost growled at her.
'So he gets away with it, and I get in trouble?'
'I was only planning on giving you a quick telling off, but your attitude has convinced me otherwise. Detention mister Black with me at 7, and should another incident occur I will write home your parents.' Sirius paled. They were bad enough already. But if they discovered his attack on a Slytherin ... they'd make life hell for him. Even more than it already was anyway.
'You will share the detention with your cousin, miss Black. Do not be late. You are excused.' Sirius nodded, wanting to argue but employing a rare bout of common sense and following Moony's tactic of saying nothing and trying not to be noticed. Not something that came naturally to him. Detention with Bella. He'd have to play it up with James and the others, but honestly he didn't mind. Bella might be eccentric but she made better company than some of the Gryiffindors. Not that that meant Slytherin's were better than them. It was the other way round.
Just a little shot of Sirius growing up at Hogwarts. I've always imagined Sirius being a Slytherin at heart, but a lion up front. And being friends with Bella, while ironic, is another of my favourite plots, but not a Bella under the Imperious curse. She's still evil.