Well, at first, there would be disbelief among many Slytherins. But once they realize it's true, a few of the more empathetic ones might actually feel sorry for Harry. They could start to see him as more than just the 'Boy Who Lived' but as a fellow student who has been through a great deal of pain. They might even try to befriend him secretly, away from the prying eyes of their housemates who might not be so accepting.
The relationship between Slytherin and Gryffindor houses could shift. Slytherins might become more protective of Harry, which would be a huge change from the usual rivalry. This could lead to some Gryffindors being suspicious of Slytherin motives.
You might want to check out 'Secrets Unveiled'. In this story, Draco discovers Harry's abuse by accident when he follows him to an abandoned part of the school.
Well, some Slytherins might initially be skeptical. They are known for their cunning and might think it's a ploy. But if the evidence in the fanfic is convincing, some could feel a strange sense of sympathy. After all, they are not all bad. They might start to see Harry in a new light, not just as the Gryffindor hero but as a victim too.
They would likely be shocked and angry. The Weasleys are a kind - hearted family. They might immediately take Harry under their wing more protectively, like Molly Weasley would probably start fussing over him even more, making sure he has enough to eat and a warm place to sleep.
Well, the goblins could start a revolution of sorts. If they find out Harry is abused, they might see it as a sign of the general corruption and unfairness in the wizarding world towards those without power. They could use Harry as a symbol of this oppression and start demanding changes in the way the wizarding world treats its children. This would likely lead to a lot of conflict with the wizarding authorities, but it could also bring about much - needed reforms.
In some fanfictions, the Death Eaters might see Harry as a tool. If they find out he's abused, they could try to manipulate him. They might think that with his power and now his vulnerability, they can turn him to their side, using his anger at his abusers as a driving force.
They might be quite surprised. Some Slytherins could see it as a chance to taunt Harry's friends, while others might actually feel a bit of sympathy, even if they don't show it openly. After all, bullying is not something to be taken lightly.