In this kind of fanfiction, Hogwarts would likely react very strongly. The portraits on the walls might spread the word, causing a stir among the students and staff. Harry's housemates in Gryffindor could band together to form a sort of support group for him. The Weasley family might even offer to take Harry in permanently. The Hogwarts elves could also play a role, perhaps making sure Harry always has his favorite foods to make him feel better. And as for the professors, they would no doubt adjust their teaching and mentoring of Harry to help him heal from the abuse he endured at the hands of the Dursleys.
Well, first of all, the students would probably be shocked. His friends, like Ron and Hermione, would be extremely angry at the Dursleys. They might try to comfort Harry more often. And the school itself could start providing special resources for Harry, such as counseling or extra protection spells. Also, there could be a big change in the way the wizarding community views Harry. Instead of just seeing him as the 'Boy Who Lived', they would see him as a victim who needs support.
In some versions of this fanfiction, it could be Neville. He also had a difficult family situation, so he might be more attuned to signs of distress in Harry. Maybe he notices Harry's worn - out clothes that are too big for him, which could be a sign of neglect, and then starts to piece together that something is wrong at the Dursleys'.
Well, if we consider the 'umbridge finds out harry was abused fanfiction', it really depends on the writer's take. One possibility is that Umbridge, being the power - hungry character she is, might try to cover it up to maintain the status quo at Hogwarts. On the other hand, she could also see it as a way to discredit Dumbledore, thinking he failed to protect Harry. There's also a chance that the discovery could lead to a change in Umbridge's relationship with Harry, from being a tormentor to an unexpected ally, though this would be a huge shift in her personality.
One possibility is that Narcissa would try to use her influence within the pure - blood society. She could spread the word among the more reasonable pure - blood families about Harry's situation. This might lead to some sort of support network being formed for Harry, even if it has to be done in secret to avoid Voldemort's wrath. She might also try to subtly nudge Draco into being nicer to Harry, which would be a big change in their relationship.
In this kind of fanfiction, Cedric might feel shocked and angry. He could decide to protect Harry from further harm. Maybe he'll try to find the abuser and put a stop to it.
In such a fanfic, Snape might initially be shocked. Given his complex relationship with Harry in the original series, he could struggle between his old grudges and his sense of duty. He might take it upon himself to protect Harry secretly at first, perhaps by using his position at Hogwarts to keep an eye on Harry's abusers. For example, if it's the Dursleys abusing Harry, Snape could find ways to make sure they don't go too far without revealing his true intentions.
In this kind of fanfiction, there could be a lot of shock and disbelief among the characters at first. His friends might feel guilty for not noticing the signs earlier. They would probably become extremely protective of Percy.
It could be very distressing for Harry. He might feel a sense of responsibility to do something about it, like trying to find the author to offer help or support. Maybe he would also be shocked at how some people can imagine such dark things happening in Hogwarts.
In some fanfictions, Harry might discover that students at Hogwarts are being abused by certain dark forces or corrupt individuals. For example, there could be a fanfic where a new dark wizard has infiltrated the school and is using dark magic to mentally or physically abuse students. Harry could discover this through seeing the strange behavior of the victims or finding some clues around the school.
Well, one possibility is that Artemis would become extremely protective of Percy. She might use her powers to shield him from any further harm. She could also seek revenge on those who abused Percy. In the world of fanfiction, this could lead to some really interesting power plays and conflicts.
Well, Draco might initially be shocked. He could choose to confront Ron about it. Maybe he'd secretly help Harry by providing some kind of protection or resources. After all, Draco has his own sense of justice, even if it's different from the norm.