"I cannot say that I was a particularly diligent student, especially during the lower grades"

Koichi Tanaka's quote recommends that he was not an especially diligent trainee throughout his lower grades. This might mean that he was not as focused on his studies as he need to have been, or that he was not putting in the effort needed to attain great grades. It might also imply that he was not taking his research studies seriously, or that he was not benefiting from the academic opportunities available to him. Whatever the case, it is clear that Koichi Tanaka was not a particularly thorough trainee throughout his lower grades. This might have been because of a lack of motivation, or a lack of understanding of the significance of education. Whatever the factor, it is clear that Koichi Tanaka was not as concentrated on his studies as he should have been during his lower grades.