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School grades are useless arbitrary numbers.
Because they represent a single, subjective point of view, strongly biased by the teacher's personal preference towards specific solution to an exercise.
To make them relevant, a student would need to be evaluated by a statistically significant number of teachers, say 10 at least. But that's not practical.
The most intelligent people have mediocre grades in average. But again, so do the rest of the people. Hence my point.