[DiogenesKynikos]

Grades from different schools are not equivalent.

It's extremely difficult to compare grades from hundreds or thousands of different schools. You have to really know how rigorous each individual school's grade scale is.

[elgenie]

Grades from different schools are not remotely equivalent in representing material learned.

However, they are comparable with respect to the hoop-jumping conscientiousness of the students involved, and there's a decent argument that it's the latter is what matters for successful completion of college.

[DiogenesKynikos]

Conscientiousness is not all that matters. Schools can vary widely in their level of academic difficulty.

Top students who come from one school (to take one extreme example, Stuyvesant high school in New York, which has produced four Nobel Prize winners) may be on a completely different academic level than top students from the average school.