How children are seen in the classroom affects them a lot, whether the teacher or child knows it or not.
Have you ever taken a class where the teacher is very positive about your abilities? I know I have. It makes you feel great and allows you to gain confidence and live up to your full potential.
Now think of a time where someone limited you, and didn't take your ideas seriously. It felt bad, right? Conversely, when a child in the classroom is not being valued for what they do and limited because of their perceived "ability", they will suffer.
The fact of the matter is that if you limit a child's capabilities due to perceived "fluency", you are unnecessarily closing opportunities for them to naturally develop their abilities. So next time you stick a child in a bracket, think again.