Identification of gifted children within school systems usually involves three elements that overlap ... In this widely used model, developed by Renzulli and Smith (1980), a child is identified to participate in a gifted program only when these three element overlap substantially. Another way of stating the Renzulli model is that superior ability, itself, is not enough - there must also be high motivation to use that ability, and it must be expressed in creative ways, or to an unusual degree. Because it insists on the clear expression of giftedness, use of the Renzulli model overlooks many gifted children who, for a variety of reasons, are unable or unwilling to demonstrate their talents in the ways being measured. For example, gifted children with cerebral palsy, learning disabilities or hearing and vision impairments are often not identified as gifted.
-Webb, Meckstroth, and Tolan, "Guiding the Gifted Child: A Practical Source for Parents and Teachers."
I'm not sure why teachers think that gifted children should also be highly motivated (to follow the teacher's agenda) or that a child must be creative to be identified. In fact, I'd even go so far as to say that the creative aspect needs to be creative in a way that is appropriate to school employees who are not familiar with giftedness.
What is more disturbing, however, is that if you have a child who is lacking in either creativity or motivation, often proof that a child is gifted is viewed with disbelief. I'm not sure if these children create such a negative impression of themselves that teachers are unwilling to believe, that giftedness isn't as important as conformity for these teachers, or if the teachers simply cannot move beyond their preconceived notions of giftedness including creativity and the even more important motivation factor.
Once you have reached that point, teachers no longer believe parents. They will only believe people who have letters after their names and can write big, fancy reports. Even then, they usually have a lot of reservations about viewing a child as gifted.