That is an interesting question. I think that good teachers do teach at the majority's proximal development level a good part of the time. It is much harder for advanced kids to get their share of proximal development teaching at school.
As far as schools "underestimating" the child's abilities, I think part of it is due to the lack of interest and perceived relevance on the part of educators. For example, it doesn't matter that your first grader can read Harry Potter for fun since he isn't likely able to compose analytic answers to literary questions required at the 5th grade reading/language arts classes.
Independence also appears to become very important by third grade so that if the child can't produce/complete work independently, it doesn't really count in some (most?) teachers' estimations.
