I agree with Bookworm about highschoolers doing more on their own as well as not necessarily doing everything in order every time. In PS students have electives in high school allowing them to pick what interests them after the requirements are filled. I think that sometimes we can be too hands-on and not let our kids have enough ownership in their education if we are not careful. Ideally by the time high school rolls around we will be able to say to our kids "Here's what you need, here is what else you can do, let's work out a plan together to make this happen." Not quite sure how we'll do it all when we get there, but I do want my boys to be able to say "ok, I don't need history in 12th for what I want to do, I'd rather do a computer course" or whatever, or "I have my required history, I prefer middle ages to ancients, lets do that again."
I think the solution to this question will be different for evey family. Some will say "Ok, lets do mod 5 & 6 American history this year, but we'll skip world history this time around." They might condense ancients to one year or two, splitting, with each mod taking 24 weeks. Maybe your child has a suggestion you haven't thought of...