  Harry actually didn't know if someone was going to attack him, but Ron was adamant that someone was going to make a play. Hermione believed that people barely knew how to fight, but they were proven wrong fairly quickly. While Slytherin had a training area in their Common Room, there was also their was a whole corridor blocked from every one else but the Slytherins. They called it the Dueling Hall, it's where people decided to settle their grievances and it was quite popular.