Why won't my parents leave me alone about my grades?

Why won't my parents leave me alone about my grades? Like sure I may not give my whole effort in school. I miss an ignment here and there, but overall I still make A's and B's so it's not like I'm a bad kid. They keep telling me about how "I don't give a **** about school." and "I know you have it in you so we are trying to push it out of you by grounding you." Which I think both of these claims are bull ****. First off I do give a **** about school, I just sometimes get in a mood where I refuse to do anything, including my homework. Plus the second statement is plain out crap because all that does is demoralize me and make me all that more rebellious towards doing my work. Also I just got cussed out by my dad not even 5 minutes ago because this time I truly didn't know we had homework one day in English and missed that ignment. That was the first ignment I've missed in any of my cles all year! I'm just sick and tired of them harping on me for my grades. Plus my dad didn't even get ******* A's and B's in higschool, he also got plenty of C's so why doesn't he ******* understand? Please tell me how to get them off my back, and don't say "Just do your school work!" Because all that does is make them turn their attention towards how I only get B's on tests and not A's and Im tired of it.