I find it interesting that most conversations about how very able children are being 'held back' or 'frustrated' by school revolve specifically around maths (and, to a lesser extent, reading in the early years, with its seductive language of 'bands' and 'levels').
It seems to me that the (genuine) frustration experienced by parents and/or children about maths has quite a lot to do with the rather odd way it is perceived as a subject; and consequently, taught. The belief that one either has a 'maths brain' or doesn't; that one is either 'good at maths' or not - and that this is evident from a very young age - is widespread... but not supported by the evidence. And it's not just parents and children who believe it - many teachers do, too; it's woven into the way we structure the curriculum, indeed, with a focus on ability setting, and very narrow summative assessment.
The fact is, being able to score highly on an academic test really isn't a great marker of how good a mathematician you are. You only need to look at 'Hannah's sweets' to see this - a whole cohort of students, many of whom would have been described both by their teachers and themselves as 'high achievers' in maths, flummoxed by a problem that wasn't presented in exactly the way they'd been taught. They had the skills they needed, but little or no experience of using them in creative and flexible ways.
The shift away from pushing through 'levels' towards achieving 'mastery' in the new curriculum is a positive one, imo.... the problem is, many schools have a very poor grasp of what 'mastery' means, and how to provide 'enrichment' work that genuinely allows all students to progress.
No child need ever meet a 'glass ceiling' in maths, because the world of mathematics is full of open-ended, inspiring problems to explore, whatever skills one has yet to be taught. We don't talk about a 'glass ceiling' at KS1/2 for history, or PE, or art... because in those subjects, we're not obsessed with levels, and just how early it might be possible to take a GCSE. Similarly, no child need to be written off as 'bad at maths' from the age of seven, and limited in terms of what he/she is 'allowed' to learn as a result.
I'm not saying there isn't a problem at the moment; and I'm not saying that it can be solved by a mind-shift from parents alone. I am saying that it is by no means restricted to apparently 'able' children'; and I am suggesting that a massive alteration in society's attitude to maths, and schools' approach to teaching it, is required.
I thoroughly recommend reading 'Mathematical Mindsets' by Jo Boaler (every teacher should certainly read it!); and encouraging children - whether 'g&t' or not - to visit wild.maths.org/.