The Weasleys are likely to be horrified. They are a warm - hearted family. Mrs. Weasley might immediately take on a motherly role towards Harry, perhaps making him his favorite foods and fussing over him. Mr. Weasley could become very protective, ensuring that Harry is safe from any further harm. The siblings, like Ron, might be angry that Harry had to endure such abuse and vow to always have his back.
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.
There could be a plot development where the Weasleys' discovery of Harry's abuse leads to a rift in the wizarding community. Some might not believe Harry was abused or might try to cover it up. The Weasleys would then have to fight against this disbelief and secrecy. They could also form alliances with other families or characters who believe Harry. In the meantime, Harry might start to develop new magical abilities as a result of the love and support he gets from the Weasleys. This could be a turning point in his growth as a wizard, as he learns to use his magic in a more positive and confident way.
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.
In the fanfic 'Malfoy's Revelation', Malfoy is paired with Harry for a project. As they spend time together, Malfoy notices some strange behaviors from Harry, like flinching at sudden movements. After some snooping around, he discovers that Harry has been abused. This new knowledge makes Malfoy more empathetic towards Harry and he tries to help him in his own way, which includes standing up for Harry against some of their mutual enemies at Hogwarts.
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.
It could be one of his close friends. In many fanfics, friends often play a crucial role in uncovering such secrets. They might notice Percy's unusual behavior, like being overly jumpy or withdrawn, and start to suspect something was wrong.