What Amanda is trying to say, I think, is that this RPG - Mystical Hogwarts - is set during Harry's 7th year. All the classes we're taking and all the RPing we do has to be in context of Harry's 7th year. This is because that book hasn't come out yet, but since we've read the others we still sort of know what's going on.
You will have to talk to Brooke about your character. She would be the one to change your username to Harry Potter, and I don't know for sure about the teaching part, but according to the setting and the rules of this RPG, if you take on Harry Potter you're a 7th year Gryffindor.
Other than that... I don't think there is anything. I would advise getting your character straight before you start RPing, because if we've got a Hermione in her 7th year and then you, Harry, as a professor... well, things could get really messed up. So just make sure you have everything settled before starting.
I hope that made sense.