Unless they're deliberately lying, people always believe they're saying what they mean.

I don't think this is true at all. I think such people believe they're saying something close enough to what they actually mean, and that social conventions don't require them to take care to make their language as unambiguous as possible. This last part is the problem.

in the teacher's head, as in those of many other commenters here, this particular example is sufficient to be clear.

Again,"sufficient to be clear" is not the right criterion; the right criterion is the ideal of "no possible way to misunderstand". (Achieving that is impossible; how much less possible is it when they aren't even trying?)

What you appear to mean--at least, what people tend to mean when they say that--is "people should say things in a way which is immediately clear to me." I hope you see why this is a tall order for people who may not know you or understand how you think very well.

This makes it sound like "clear to me" is a highly idiosyncratic criterion. There is such a thing as objectively less ambiguous language.

The basic issue here is people not thinking carefully enough while they're speaking. It's really a question of quantity of thought, not style of thought.