Oh and I should say (since I didn't answer your second question!) - I don't know if there's a predictable pattern of slowing down, but at some point the grade levels aren't the important measure. Once you get into high school material it's more about mastery of the topic (Algebra, Biology, US History) than about grade levels. And also about then it seems that test scores matter a whole lot less, and curriculum (transcripts) mean a whole lot more. At some point I expect DS will be bumping up against the ceiling of the ACT, and from then on we'll be testing only because of the state requirement... but we may do SAT subject tests and AP tests to show mastery (and substantiate "mommy grades") as we finish different courses.
