How do I tell my parents?

I'm in 8th grade and I found out 2 days ago I couldn't graduate because my grades are bad. I don't really care that much about the promotion, because I know I understand what I get taught in class. I'm just to lazy to do it. I never got lower than an advance on the CST's and I never failed a quiz/test. What bothers me the most is that my parents are all excited because they think I'm going to promote. And I just don't know how to tell them. I'm going to be going on to high school, but that won't matter to them. I don't want to tell them because they'll start calling me stupid and stuff like that.