@Xystus I think it's supposed to imply that if your
best score,
ever, in any subject, is a 79, it's likely you're just average overall. His point is that he never attracts attention - he's not good enough to be singled out for praise and become known as someone who could help others study, and he's not bad enough to have to attend supplementary lessons, where he'd get to know students who are bad at the same things he is bad at and they could help each other study.
It's also possible that he is actually above average - if his average grade is like a 65 and the class average is a 50 - but because he never talks to anybody else, it's his pessimistic
belief that he's just average. His self-esteem sucks.