It's a decent story, so far, but you've changed the characters quite a bit. I think in OotP, none of the professors were willing to remove "SNEAK" from Marietta's forehead because of her betrayal - it hardly seems like the professors would so staunchly support thieving students without due consideration. Furthermore, it's foolish of Harry to go so gung-ho about changing attitudes - he's making more enemies than friends, rather like Snape.
Author's response
Ironic, isn't it? Or should that be hypocritical?
They refuse to help Marietta after she betrays the DA, the help fight Umbridge after dumbledore is removed, yet they do nothing to help Harry in CoS when the whole school ostrasizes him for being the Heir of Slytherin even though DD and likely the whole staff know that he isn't, nor do they help him in GoF when they all know someone else put his name in as part of an attempt to kill him, nor in HBP when Umbridge begins her detentions, they believe DD when he says V is back but do nothing to help Harry when he is being crucified. It seems to me they only get involved when someone else points out that it is too late. BJH