Now he admits he left the room before Morrison even arrived and felt terrible about his lie.
"I think I did it because I wanted to be in the classroom so badly that I kind of put myself in that situation and what I would have done," he told CBS News correspondent Cynthia Bowers Thursday.
"He came in the room with a gun and shot the gun on the ground and said 'Get up on the (black)board or I'm going to shoot you," Grigg said earlier in the day on The Early Show. "He went one by one and told us, 'Hey, you could stay in the room, or you could leave.' And he got to me and I told him that I didn't want to go, that I wanted to stay."
His mother, Larina, had told the Rocky Mountain News her son was given an ultimatum.
"The gunman swung around, put the gun right in his face and said, 'You need to leave.' "
Later, she told the Rocky Mountain News she didn't know why her son had lied.
"Cassidy has never been dishonest in his life, but in this matter he wasn't truthful," Larina Grigg said. She said he "wanted to be a hero" but "got caught up in the chaos."