Oh trust me, they try. You should have seen the look on my high school principal's face when I asked to take physics as a sophomore (after all, it was a senior course). Eventually they settled on the excuse that I hadn't taken algebra II, but it was agreed that they would let me in if I dual enrolled at the local community college and took an equivalent level math course. After getting the highest grade in the class (and teaching it one more than one occasion), I was told that I would have to take algebra II the next year. Their stance was that, while I had already covered all of the material, algebra II was a graduation requirement which could not be met outside of the high school. I would not be allowed to graduate without retaking the class. Thankfully I got out.
In my experience (mine and people I know), it's not that gifted kids don't try to get ahead, it's that they are often actively prevented from doing so.
It's amazing how often I hear people refer to Matlab as a language (mostly engineering professors).
My roommate is in religious studies, so I always referred to my C class as "Old Testament"
In my experience (mine and people I know), it's not that gifted kids don't try to get ahead, it's that they are often actively prevented from doing so.