Guinness knew perfectly well that he wouldn't pass if they didn't work together as that was part of it. If the professor found out they didn't work together on it they'd both fail, so really it was in both of there best interests to work together. Besides, she didn't need to get him in trouble. She wouldn't really be able to get him back if she got him in trouble.
Besides she wasn't scared of him. She wasn't afraid of what he could do to her. She just looked at him still, she couldn't do anything else but she had no expression in her eyes. She kept her thoughts calm, it wasn't the end, but why not let him believe that it was.