Neville had forgotten all about this meeting. So, when he had seen everyone else getting up, he couldn't figure out why. Then it hit him! OH YEAH!
Jumping up, he ran toward the door, stopping right outside to calm himself. He hoped he wasn't the last one. As he walked in, he saw that he was really among the first and made his way to one of the armchairs hoping to stay out of trouble. After all, when the Headmaster called a meeting it always meant trouble right?