Because teachers will often award marks based on neatness, and how pretty the work is.
Because gifted children who think outside the box do not give the "right" answer.
Because gifted kids often think that the work is "stupid" and try to get through it as fast as possible just to be done with it.
Because a child's ability to sit still affects the teachers perception, and gifted kids are often restless and even downright annoying.
Because some of the teachers are intimidated by having a child in the class who is smarter than they are, and points out their errors.
Because the ability of gifted kids is often not measured well by the school, which relies on learning by rote and regurgitating information.
Not to complain about all the teachers - some have been great. But we have had these issues with both my kids.
These had definitely been our experience in the lower elementary grades in particular.
For all the reasons listed above and elsewhere on this thread, even the self-disciplined and motivated gifted kid can�t distinguish them self from the rest because the expectations are set low enough that the majority can meet or exceed them.