I can sympathize with teaching or being a tutor. I taught freshman students in an introductory engineering class. They struggled with simple word problems that 8th or 9th graders should understand! That was 20+ years ago, and I see no improvement since, and it reflected a problem I had identified when I was in high school 30+ years ago. (When I was a senior in high school, I got pulled out of my AP chemistry class for a few days to substitute teach the regular chemistry class because the teacher was ill.) The educational system in the US is fundamentally flawed to the point that a majority of students fail to achieve their full potential.
However, with respect to the 12th grader, how does one differentiate between innate capability and the achievement (or lack thereof) of the educational experience. Perhaps she was never challenged or never taught in a way that was conducive to her learning. Clearly some people lack the ability to learn, but most do have the potential, although it varies widely. Then we must consider that different people learn differently, and this reflects on one of the greatest failures of the US (and perhaps elsewhere) educational system - that being 'one size does not fit all'. In general, we use an assembly line approach to education that attempts to move everyone at the same rate - and that doesn't work. Each student needs a customized education!
My kids needed help outside of the normal class, and we had to go to great effort to get that supplemental help. My wife works with kids who need extra help in school, and that appears to be an exception than the norm, and it seems to work on a school by school, district by district basis.
Now getting back to the OP, I think it shows that folks with relatively average (not exceptional IQ) can achieve great work, and it does not require a high IQ. Having a high IQ may be sufficient, but it's not necessary to achieve.