Yes you are missing something, in fact a lot. SATs test have been used over the years to score political points and have become less and less to do with the educating children in a stimulating and creative way, rather as a means of forcing children (and teachers) to jump through hoops/pass exams. Headteachers and teachers realise that they are accountable and that there has to be a a way of measuring a child's educational progress, indeed every school has to already have a rigourous system of continual assessment in place. The unions have been attempting to enter dialogue for up to six years now, but so far they have refused to adapt or change the process despite the children/teachers of Scotland, Wales and NI ditching these test years ago. Teachers are paid to teach and the vast majority of them do this essential and demanding job well, but they are not unthinking "sausage factory" mentality workers, they are continually encouraged to inovate, stimulate and improve their practice, not simply blindly follow orders. In order for this boycott to come about has taken these same free thinking people years to make a stand. Anyone who has had an eleven year old go through the system of SATs know that the pressure to perform has become about statistics and not children. If you read the Mail you will get the impression we should all be good little workers and see education that is "done to someone " not "with them."
Read this if you genuinely want to learn more, it's a little less black and white than you may think.
http://news.bbc.co.uk/1/hi/education/8623363.stm
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No, they are assessed again before Christmas in every secondary school. Children in the UK are the most tested children in Europe!