Rewrite of Brotherhood, Ron went to Professor Snape not McGonagall.
I've always thought that the boys' discussion at their first Welcoming Feast should have raised alarm bells, so here we go. I've kept things canon-compliant/plausible where possible.
Disclaimer: As the name of this site suggests, this is fanfiction and I own nothing.
"Enter." At Severus's drawled permission the owner of the timid knock slips into the room, his orange hair bright in the dim light.
"Mr Weasley. And what pray tell brings a Gryffindor to the dungeons?"
The boy comes closer to stand nervously before his desk. After a moment of fidgeting Severus sees him take a deep breath, obviously gathering what little Gryffindor courage he possesses.
"Well, you see, I was going to go to Professor McGonagall, after all she is our Head of House, but I never see her in the dorms or common room, none of us do, and then I heard that you were the one to come to in these sort of situations, and, well, I need to know…" His babbling trails off as his courage fails beneath his professor's scowl.
"Mr Weasley. Get to the point, or get out and stop wasting my time." The threat seems to work.
"What's the definition of abuse?" The first year blurts out the words, his face turning red to match his hair.
"Why, in Merlin's name…" He looks at him strangely; why is a Gryffindor, a Weasley, asking him this question? The one topic that will cause him to put aside his snarky attitude and respond in all seriousness. His blood chills as a thought crosses his mind. Surely the boy can not be asking for himself? Surely he is not being asked to intervene in the family of red-headed terrors? He shakes his head and turns his attention back to the nervous child before him, he is getting ahead of himself.
"Very well, Mr Weasley. I will answer your question, but then I do expect you to tell me why you find it necessary to know. There are four main categories of abuse; physical, psychological or emotional, neglect, and sexual. The non-accidental infliction of injury or pain. Verbal assault, threats, intimidation, or emotional manipulation. The withholding or lack of provision of necessities both physical and emotional. Any sexual contact with a minor or without consent."
The boy nods to himself and Severus marvels at the expression on his face. Gone is the earlier nervousness, instead he is serious, thoughtful, taking in his words and comparing them to some previous concept. His manner is completely different to what he has seen so far, and he waits to discover what has prompted the change. Blue eyes meet his and he suddenly finds himself wanting to look away. He doesn't want to hear what he will say next; it is a Gryffindor problem, not his. Yet the boy's expression says otherwise.
"Professor, I have some new definitions for you. Neville, Seamus, Harry. Neville, Dean, Harry. Possibly Dean, probably Neville, definitely Harry. And, bloody hell, I hope not Harry."
The teacher in him notes his language, but the rest is reeling. When he saw the direction the conversation was taking he knew it would be bad, but all of them? And Potter. Perfect Prince Potter. He is supposed to be pampered and arrogant like his father, not… this. His mind shies away from the thought but runs into another equally uncomfortable. If he never saw it in any of them – and even this early in the year, he should have – what else has he missed?
"How…?" For once, his mind is too incoherent for words, but the Weasley boy understands.
"The thing you have to remember, Professor, is that I grew up with five older brothers, including the twins; I know how to read people. Besides, we talk, in the dormitory; most of them don't really understand that what happens is wrong." His voice is hard and Severus has trouble remembering that he is just an eleven year old.
"Dean, you know, is a muggleborn. They didn't know what was happening when his accidental magic started, his parents thought he was possessed or something, tried nearly everything they could think of to fix it. He hasn't gone into a lot of detail, but I know that discovering it was magic really didn't help anything. He almost had to run away to come here." This is not a story Severus has heard often working with his mostly pureblood Slytherins, but it is more common among those from other Houses. He makes a mental note; this is a situation that will need to be resolved before the summer holidays.
"Seamus had it slightly better, at least his mother knew about magic and was able to help him, a bit. Didn't help with his father though. He's muggle; she couldn't tell him about magic until after the wedding, he didn't take it too well. Things are pretty much fine as long as his Da can forget about it. It seems that he's learnt how to feel his accidental magic building, and to suppress it until he can release it again out of his Da's sight. Unfortunately, the results tend to be slightly explosive…" Severus nods, that would account forthe boy's pyrotechnics aptitude then. He tries hard to avoid focusing on the rest of the explanation, the child's situation far too familiar for comfort.
"Neville, on the other hand, has the opposite problem. For years his family thought he was a Squib, and kept half-killing him to try and draw his magic out. When they finally did, it was by dropping him out of a tower window; they were more excited about the magic than they were about him surviving! Now, he's still not really convinced he's a proper wizard and is terrified of disappointing people and not living up to his father's example." He winces. Blasted Gryffindors. That… explains a lot about the boy. As frustratingly inept as the child can be, even he knows that approach can only have made things worse.
"As for Harry, I don't even know where to start with those bloody muggle relatives of his. I mean, his bedroom up until a month ago was a cupboard; literally! From what I can tell, if you think human house elf working for a Death Eater family you get the idea. And the worst is, he seems to think he deserves it." No. That is not Potter's son, Lily's son. It can't be...
"Professor, those are my dorm mates. Four out of five. Think about it; that is the boys, of one year, of one House. What is wrong with the world?"
Severus feels his preconceptions crumbling around him. Thinking of the list of students from his own House, and those who have been uncovered in Ravenclaw and Hufflepuff, for once he can only agree.