Fantastic! This is the type of story that I reread paragraphs over and over again because i enjoy them so much. I love, love your Snape. Wonderful job on him and his dialouge is very true. Everything he says is very entertaining. There are erros in this chapter too you need to go back and take a look at. :) Many. Please try to correct them, otherwise it hurts the story.
You may want to reconsider this: Snape sighed as he grabbed one of Harry’s wrists to keep the boy from punching him. This is the second time I've seen an action refered to only after it happened, passively. In this case, Harry trying to punch Snape. It makes it seem delayed. Afterwards you used it again.