Stop for a minute, and actually picture teaching a class. Unless you have forgotten to mention that your child is at private school, I'm guessing she's in a class of 25-30 kids with one teacher and maybe a TA (although maybe not, or maybe not all the time).
You say she "should not be measured or compared to anybody else", but when you are trying to teach 30 kids, if you can't measure what they know and compare it you just teach them the same. Imagine maths
We're going to teach 2+2=4 - four things happen as a result:
- Some children have no concept of numbers - they get muddled and frustrated because they don't get "2"
- Some know 2 and 2, but "+" and "=" baffles then and they get stuck
- Some know 2, pick up the concept of "+" and "=" quickly, and get there
- Some learnt 2+2=4 when they were 3 and are bored stiff for the whole lesson.
Now, same lesson, but we start by comparing the children first. We compare those groups of children:
- the first group we know is going to struggle. In fact we know they are not ready to learn 2+2 = 4. So we set them up with a teaching assistant, paid for by the funding we get through most of them being on the SEN register, to learn what 2 is.
- The second and third group are maybe taught together and get there with 2+2=4
- The fourth group are taught something else entirely.
Without any measuring or comparison, there are no groups. There are no different levels. There is one thing being taught and the only differentiation is by outcome - children who learn it, and children who don't. Without children being labelled as needing more help, there is no knowledge of support needs, and no money to employ extra staff. So all 30 kids get taught one thing by one teacher. Those in the first group try to learn as best they can and most probably most don't.
The fact is that if we don't measure & label and then use that labelling to generate money to get extra staff and resources, some kids are constantly being taught stuff they know, while others are being taught things that are too hard. Embrace the labelling, embrace the support and embrace the fact that your child is being helped and encouraged to fulfil their potential whatever that may be.