Missing my exam?

basically is it okay to miss a retake AS exam. I am not prepared for it, because i wasn't aware that my teacher had put my through for the exam. He briefly asked my class a few months ago if anyone was interested and i said i was, but he never came back to us, or told us about revision lessons like he promised. So i assumed that i wasn't even doing it, until a few days ago when i saw the exam on my timetable. I got confused. So i am thinking of not going but whats the rules with that, am i allowed to just not show up? or do i have to email my teacher (tbh there is no time the exam is tomorrow) but you get me, what would be the best thing to do in this situation? will i still get graded or can i ask them to dismiss the whole thing?