This is not exactly true. The best public schools (e.g. in Virginia, Thomas Jefferson High School for Science and Technology) will have a relationship with the admissions committees at the best colleges in the area and nationally. In these cases they really do know and care about how the grading system is applied. For good private schools, the effect can be even greater.

This insight doesn't apply to all public schools, though. If you are a standout student in a terrible off-radar public school, you will have to work harder through other channels than the reputation of your school.