I teach in a FE college, and I think you have already been given some good advice. To add a couple of thoughts...
Don't get too worked up about it, as it is not the end of the world. As others have pointed out, whilst some subjects have tiered papers not all do, and so in those subjects (History, Religious Studies etc) it is perfectly possible for your daughter to still come out with an A* at the end of it. Only some subjects have tiers. It might be worthwhile asking the school which ones have tiers.
Also, when I interview students (and I speak to hundreds each year), they often talk about moving tiers. Often this is downwards (I hear it most on maths - that students have been moved down to foundation tier to ensure they get their grade C in maths), but occasionally I interview someone who has done well and been moved up a tier. This can't be done too late, as there will be gaps in their knowledge, but it is definitely worth asking the school whether it is possible to move up a tier if your DD proves herself when on the GCSE course.
Finally, whilst not ideal, it is increasingly common at our college to see students resitting their GCSEs in the first year and then moving on to A levels the following year. This is often students who get grade Ds the first time round. We only used to have a few of these students, but every year it is more and more. As you say, some students are just late developers and need that extra time to mature before getting their GCSEs. It doesn't mean failure. I have seen one student do this and go on to get Bs in their AS levels. It doesn't always mean the end of the road.
In your situation, I would want to have a meeting with the school, as whilst most schools do act in the best interests of the student, my years in teaching have shown me that the pressure of getting the golden 'A-C' grades does mean that some teachers act politically rather than what is actually in the best interests of the child. I have seen examples where students have been encouraged to sit foundation papers (to ensure the golden C grade is achieved) rather than trying for a B grade on the higher paper and risking not making the mark... Sometimes it is the best thing for the child, sometimes not.
Also, I note that your child is going to do a BTEC in science... If you're looking at going on to college, I would just check how many points BTECS are awarded. Not all colleges accept them / count them for as many as the schools claim that they are worth. For example, I know of one school where they tell the students that one particular BTEC is equivalent to 4 GCSEs, but the college the students subsequently go on to (not my college btw), only accept it as one. It is worth checking!
And good luck
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