Basically, high grades aren't the end-all, be-all of school. They aren't a way of defining ones' worth. The analogy of treating school like a job really applies here. TFA also points out the risk of bad grades.
I should point out that TFA's advice works for conventional high school, but less for prep school. I had so much homework in prep school that it was nearly impossible for me to pursue the kinds of things TFA advocates for. Had I known better, I would have insisted on going to public school. (But all my friends were going to prep school; another mistake from TFA that I fell into.)
