You just give multipliers for advanced classes then. That's how my high school calculated GPA - if you took all "A" level classes your max GPA was 4.0, but if you took AP classes and aced them you could end up with a 4.3 or something like that

That's how almost all highschools and colleges do advanced classes, Honors classes are graded on a 4.5 scale and AP classes on a 5.0 scale (and also count for college credit so you get to skip some intro classes after admission).

This, of course, leads to yet more grade inflation. Hard to compete with a >4.0 student when your school doesn't even offer advanced courses!