I think it is important that parents know how their child is doing compared with their classmates as, like it or not, not every teacher knows what is right for every child and, further, like it or not, not every teacher's style of teaching suits every child. Teachers are human beings with individual personalities, not blimming all-knowing demi-gods!
My DD2 was struggling badly with Maths in years 7 and 8 (private prep school) she was in the top set for every subject and was in the top quarter of the class in each subject except maths, where she was struggling badly and trailing the rest of the class (barring one other child) by quite a long way. She was often in tears at home over it and claimed to not understand most of the lessons that the teacher was, "going too fast".
I went into school and discussed it with the Maths teacher (30 + years experience) who basically told me that she was just plain lazy and needed to, "get her finger out" 
Cut to the chase, I made enough of a fuss to get her moved down to set 2 for maths, despite arrogant sod of a teacher claiming that if he couldn't teach her, nobody could!
Result? She blossomed under the kind and careful guidance of the set 2 teacher, who was actually a middle-aged ex industrial chemist who had arrived at the school only a couple of terms previously having just got her teaching qualification, and was really the school chemistry teacher, just taking lower sets for maths to fill in her timetable. DD's confidence was also bolstered by being one of the more able in her set rather than one of the stragglers.
When it came to the CE exam, DD got 98%, the third highest mark in the whole school, only two children from set one beat her!