Classwork isn't some static thing; historically, boys had higher GPAs than women, and I suspect you can find some subfields of study today where boys consistently outperform women as far GPA goes. The choice of what subject matter goes into the curriculum that a GPA represents and how's it's taught and evaluated is a political choice. You could very easily make GPA equitable by removing one class that girls over perform in and replacing it with a class that boys over perform in.