Having taught gifted students for 31 years, I have heard this statement from my students frequently. I have to remember that gifted people only make up 6% of the world population and this 6% has to deal with the other 94% who aren't gifted. Gifted people are 'wired' differently and have a strong desire/urge to learn and be challenged.
Most teachers are not taught about gifted differences. They feel threatened when a mere child knows more than they do about many topics. In many cases, the teacher is not gifted and doesn't understand the way a gifted child acts/reacts to everything.
In my opinion, only master's level teachers who are trained in gifted education should teach gifted students. It would help tremendously if the teacher was gifted, also. Gifted students should have their own schools where their academics and enrichment could be taught by understanding teachers.
That negative attitude towards gifted students would disappear, I think.
Hope this helps.