It's not the teachers fault (unless they created the adventuring standards, and even then the standards should be for the majority.) The problem is the easiest way to measure someone's ability is the level of skill they learn and not their proficiency in the individual skills, and so the adventurers guild base their judgement on that while not being flexible enough to increase their judgment on the rare case of people learning multiple different class skills. And our MC is too dense to judge outside of the standards as well.
Oh, it's definitely the teachers' fault for
a) telling him he can't be the classes a-z
b) NOT telling him the things he HAS managed to learn are super powerful,
thus making him think he is really weak.
And yes, I agree he should have been able to compare with others around him, but by the time he got back from training, he'd been told so many times that he was a failure, that some regular Joe saying ML was impressive wouldn't count compared with being told by the strongest in the country that he wasn't good enough to be an adventurer.
And yes, he's not clever, but why didn't the clever masters, who even discussed him amongst themselves, figure out that he had a totally whacked self-image?
He was training with them for quite a while.
As for him disappearing - nothing surprising there. Every time he "failed", he was sent away, so of course he'd leave after "failing" the last of the 6 class training courses.
Those who think he's stupid should consider what those 6 powerful masters/teachers are.