Since I started eighth grade in September I've managed to get at least one F on my report card and I have to stay after school fairly often. My parents think I'm lazy and don't think about work, but what I really think is that the work is boring and doesn't challenge me. My mom is really going to yell about my latest report card.
There are always going to be some classes that are boring and unchallenging to you. There were for me and for most people I know. But my guess is that you are smart enough to figure out how to make it through them with a minimum of effort. Once you get to senior high, you will have a bigger selection of classes and more opportunity to take those that interest you.
Yes, your mom is probably going to yell, but you can take that if you have a plan. What is different about the classes that you succeed in? What would you have to do differently to at least pass the classes you feel are boring? Write it out, and when your mom is done yelling, talk with her as calmly as you can about why you can succeed and what you will do to pass. My guess is that your mom is scared and like most of us parents, she doesn't always know what to do. When she sees you taking responsibility, she will have to take another look at her definition of lazy.