Should I have sex with my teacher?

First off, I'm a guy and my teacher is a women. I've always had a great average in her biology class but one test that i wasn't able to finish really dropped my grade. I complained about the test after class and she offered that if i were to have sex with her she could eliminate that test score so it wouldn't effect my grade. I thought she really didn't mean that but she turned out to be serious about it. She meant real sex in her classroom after break, not just a quick one. And she even had a little affair with me that too. And It is legal, I am almost 19. Now, I'm still debating whether or not i should have sex with her. I'm already in a relationship right now so that is what's bothering me, what is more important my education or my girl? And I am also afraid that she may find out if i had sex with her. I've never had sex before but i know how i guess.... and my teacher is in her late 20's and most students including me think she is pretty. Is it worth doing this? Should I bring a condom if i were to?