wow, that's pretty (insert bad word here)ed up. it seems wrong that he is telling you that you have bad grades, but, you could try to pitch this idea to him, that people who play a musical instrument get better grades. which is true. but that probably won't work. well, this kinda goes with what chucklivesoninmyheart said, that sometimes you need to run away for a day or so for them to take you seriously. but I'm not going to recommend that you do it. now, is this like a sudden problem? or have you been fighting with your stepdad over various things for a while?