My youngest rarely complains of boredom either. For her, I think that it is a combo of being such an out of the box thinker that school is never a fit for how she approaches things so she doesn't expect engagement and being a huge extrovert whose focus is on the social aspect of school anyway.
Certain personality types are much more likely to complain than others. Ruf explains this in terms of the Myers-Briggs profile:
"When a gifted Feeling student is assigned tasks that are below the gifted student�s interests or abilities, the Feeler feels insulted and diminished. The difference here is that the Thinker thinks poorly of the teacher, while the Feeler is hurt by the teacher�s lack of awareness or concern for how the student feels and what the student needs."
http://www.educationaloptions.com/resources/resources_rufs_tips.php
