The abuser could face magical justice. In the wizarding world, there are probably spells or enchantments that can be used to punish wrongdoers. James Potter might use his connections to ensure the abuser is dealt with severely, like being sent to Azkaban or having their magic restricted.
He might be filled with rage. James Potter was a loving father figure in his own right. So, when he finds out about Harry's abuse, he could immediately seek to confront the abuser. He would want to protect Harry at all costs and might use his magical skills to ensure Harry's safety.
James might be shocked at first. He could then feel a sense of protectiveness towards Severus. Maybe he starts to see Severus in a new light and tries to find ways to help him, like keeping an eye on those who abuse Severus.
Well, James is a loyal friend. So, when he discovers Sirius is abused, he would most probably drop everything else he is doing to focus on Sirius. He'd be angry, not just at the abusers but also at himself for not realizing it sooner. He'd make sure Sirius knew that he wasn't alone and that he had someone who would always be there for him. James might also encourage Sirius to learn self - defense magic more intensively so that he could protect himself in case the abusers tried to harm him again.
One possible plot development could be that Harry first goes into denial. He has grown up thinking his situation was normal. But as he realizes he's been abused, he might start to distance himself from those who have been part of the abuse, like the Dursleys. He could then seek out help from his friends at Hogwarts, like Hermione and Ron. They would support him, and together they might try to find a way to deal with the emotional and psychological trauma he has endured.
After James discovers Severus' abuse, Severus may experience a range of emotions. He might be initially reluctant to accept James' help, given their history of animosity. But if he does accept, it could lead to a transformation in his self - esteem. He could start to believe that he is worthy of protection and care. In terms of his relationships at school, it could either improve his standing if the other students follow James' lead in protecting him, or it could make things more complicated if there are those who oppose James' new - found support for Severus. His academic performance might also be affected. If he feels more secure, he could focus better on his studies, but if he is dealing with the fallout of the abuse being known, it could be a distraction.
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.
The other students could have a range of reactions. Some might be sympathetic and try to befriend Harry more. Others might be afraid, especially if the abuser is still around. But overall, it would create a stir in the wizarding community, and there would be a general outpouring of concern for Harry's well - being.