Conjuring not hissing!

Finally the fourth year in Hogwarts had begun and the pupils returned from their vacations to their classrooms. So did Diana, a kind of special girl out of the house Slytherin. Why she was special? Because she was a halfblood girl and the others never let her forget about that. But she had got used to it, she started to separate herself from the others to didn't get in trouble. But then a pair of simblings entered her class and everything started to change. Why and how? Don't ask me, read it yourself!