You need to think

Delicious as Grace is, you're sure that you can't recover enough stamina to fuck her a third time, no matter how much she encourages you. Besides that, you remember that you share the cottage with another teacher and you have no idea when he'll be returning.

You kiss her tenderly, stroking her lithe body as you look into her happy glowing face. Your mind considers future possibilities as she snuggles contentedly. She gives every appearance of infatuation, which while it has it's points could lead to serious complications. You're also well aware of the impropriety of fucking a student, regardless of whether she's willing or not. This could not only put a final end to your career, it could result in legal consequences.

Long term solutions aside, you're keenly aware that you have to get Grace to leave before your roommate appears. Having her come out of your bedroom, obviously well fucked, as he came up the stairs would be more than embarrassing it would be downright catastrophic. The key is to ease her out without upsetting her and causing her to make some angry error in judgement that could mess you both up.