Delphinium, you have overtly, continuously, repeatedly (!) made your point: enough now, please.
Endlessly making the same point over and over is not going to help dd remember how to work out a percentage and it isn't going to help DS read.
Oh and I'm not talking about one teacher. It's a culture, I suppose.
Perhaps if you don't have a child who struggles - really struggles - with basic skills it's hard to understand, but because so much of a child's life is based around school achievement, it is imperative that you drill it into them that academic success is not the be all and end all, and their worth as a persons isn't defined by levels or by grades.
Otherwise you are setting them up to fail, not just at school but at life. You are teaching them that they have no value. You are teaching them that they only matter if they achieve the impossible. A bit like having a child who is gay - you love or accept them as they are or you try to change them. Most parents go for the latter and so did we!
Shaming DCs, humiliating them, goading them, jeering at them, doesn't inspire them, it just causes upset and discord. My kids could be the ugliest, slowest, bottom of the class, nine Us at GCSE and work in Maccys all their lives - I'd STILL love them and think they were the best! Wouldn't you?
We lost a baby, and it makes me laugh when people speak of ambitions for their kids, I've got one wish one hope for mine. Alive.