That's an interesting stat about the difference in college degrees among Republicans and Democrats. I hadn't previously heard it, but it makes sense.
You're right that anyone who finished high school should be able to write at a Grade 12 level. But with the way American education is going, I'd be shocked if a majority could. The decline in the reading and writing levels of U.S. high school students, over the past a couple of decades, is tragic.
I'd have more confidence that a university-bound sixth-former in Britain could write at a Grade 12 level, but I wonder about slippage there, too, particularly in Scotland under Nicola Sturgeon's changes in the once glorious Scottish educational system.