I said earlier there was a lot of talk of "bringing down Acacia" despite the fact that we hadn't seen her do anything than be a manipulative teenage girl. So much so, that it's basically become the main focus of... everything. We're really robbing ourselves of some interesting stuff by focusing on something like that What's more interesting, deciding we don't like her right away? Or how about actually playing through it. You know be fooled. It's more fun to slowly discover something, especially if you have to keep doubting it. 

That said, I think Acacia's charade would be more believable if she kept it up even around her own house. I mean can you really depend on Slytherins like that? By opening up to the you depend on their stereotype of being pretty much evil. But if they follow that stereotype, they REALLY can't be depended on as they'd likely sell out Acacia to save their own skins.