I know they mean well but I am starting to get frustrated ...

It is clear that they will not offer any extra acceleration or enrichment in maths until some 'authority figure' (that has to be dutch and from their official circles) tell them to do so,

They have requested the help of a special committee and for that they have to fill in a questionnaire explaining what the problem is. They gave us the form, ok good, but it is clear that they do not fully believe us. They say for example 'in the classroom X does this, but at home the parents say X does different'.

Fortunately we can participate in the next meeting of this committee (usually this is not the case), but we have to plan our strategy very careful.

Good Luck Isa.... At least they are willing to talk to you, that's at least a step in the right direction. Sometimes you just gotta hit them over the head with a board, maybe a video of what DD does at home?

I think I want to convince the school to do a 'didactic research' ( I am not certain how to say it in English) by an independent professional. I think this is like an achievement test but in Dutch of course.

In the meantime DD is disliking the school more and more

Oh well, at least she is advancing in the reading: today she just read an easy book that last week was still quite difficult for her.
(she was completely unmotivated and refused to even try).

glad to hear about the reading.
Sad to hear about the school, but it may be a giant step forward if you have isolated the problem that school hasn't yet seen reason to see things as you see them. This is a fairly easy problem to fix. Much better than, we see what you are seeing and still don't care.

I like the idea of making video recordings. If it's simpler to make up or find some work sheets showing what she can do - in her own handwriting, at home, that usually works to.
Try to think of yourself as an investigator - gathering evidence of her 'readiness level.' Sure it would be easier if she were performing at an even level across every subject, but that doesn't seem to be the usual case, does it?

Hopefully the athority figure will come through for you. But if you show him or her work sample, that will help as well, yes?

Love and More Love,
Grinity