After lunch you and your friends met up in the library as usual, taking a seat at a table near a window. Probably out of boredom, or perhaps the lack of motivation to study you suggested to explore the forbidden part of the library. It took some time to convince your friends but when you managed to finally do it and got to the forbidden library, it didn't even take 10 minutes for you to get caught, and since it was your idea you took the fall for it. That's how you ended up having to pass 2 hours in detention with Snape

You found yourself walking inside the potions classroom. Snape doesn't even look up from the book he's reading, simply muttering a "take a seat." In that dull tone of his. It's going to be 2 long hours.