I would also recommend using the Lexicon for the potion ideas.
As for the classroom descriptions, there are so many throughout the books, that hopefully we all know what it looks like now. But, I would add something about the chill in the air being a bit frostier. I have noticed when Severus is in a bad mood, whether in canon or in FF the air gets a bit chillier.
If you read Professor rated stories,
here is a very good representation of how he treats his classes. And how to actually write the classes. He is in a very bad mood for quite a while, so it will give you several reference points to choose from.
If you still have trouble writing it, just refer to it after it supposedly took place. I have some things I can't write either, but when they are needed in the story, I just refer to them in hindsight, or have someone else tell the characters what to expect.
Don't get discouraged, we all need help from time to time. Keep trying, and if it helps, try going at it from a different viewpoint, if the scene has to take place in the classroom.
I really hope I helped,
~Wendy