What year is Harry Potter poison classes in 5?

There is no mention of a "poison class" in the Harry Potter books. It is likely you are misremembering or confusing it with another subject.

Perhaps you're thinking of:

* Herbology: This class focuses on the study of magical plants, some of which can be poisonous.

* Potions: Students learn to brew various potions, including some that contain potentially harmful ingredients.

If you can remember any other details about the class, it might help to clarify what you're thinking of!