I'm just a beginner in aikido, but I did shotokan karate previously. My take on the belt system is that it is a big mistake to compare it with academic achievement. Better to go back to what preceded it--the menkyo certification, which was based on ability level and understanding of the underlying principles of the system. Shodan is not equivalent to menkyo, but the principle that it is based on ability and understanding should still apply. This won't be a popular view because we don't use ability-based systems in education in modern democracies. However, it is the most sensible system.
Mary asked: "What's a shodan?"
I asked in another thread: "How do you define 'shodan level'?"
The belt ranking system was adopted by the Butokukai of Japan for standardizing martial arts. I don't know what their original rationale was for shodan rankings. However, the original belt system was from Kodokan Judo, where the black belt was meant to indicate a student to whom beginners could address questions--one who had adequate understanding of the techniques. This is still how I think of it.