"Dad, I'd rather not discuss my GPA because the number is not the sole indicator of how I'm doing in school, or what my prospects will be. There are other things to consider, such as internships, etc.. (I don't know about law school but I assume there are other factors that would help you in seeking a career besides a standalone GPA.)
Dad, my GPA to me, is like your salary to you. I would never presume to ask your salary and discuss that. So I hope you can understand that my GPA is like my salary, and right now it's not up for discussion or analysis. I'm happy to talk about the classes I'm taking, the papers and books I'm working on and the other work I do in the program. But I won't rattle off grades. I just don't want to be measured up by a GPA."