That’s why it isn’t a reliable indicator of aptitude. A student who earns all A’s at a top level public school and a student who earns all A’s at a low level public school aren’t necessarily operating at the same academic level.
They asked what else could be used. I told them, then I explained why it wouldn't work as well as a standardized test.
And the variance can happen even in "top level" public schools, for certain definitions of "top level". I went to one of the best high schools in the nation (as rated by college acceptance rates, SAT/ACT scores, etc). There were still teachers people wanted to avoid because they were seen as harsher graders. So you can have grade inflation even at "top level" schools as private schools aren't above selling grades.
