The Department for Education (DfE) published revised league tables today (December 15), showing how the country's 11-year-olds had performed.
To reach the DfE's 'floor' standard, 65 per cent of pupils in each school had to reach the expected level in maths, reading and writing - or schools needed to be making sufficient progress in those three subjects.
In West Sussex, only 45 per cent of children were judged to have made the grade - 50 per cent of girls and 40 per cent of boys.
But school leaders' union the NAHT slammed the data as "not worth the paper it is written on".
General secretary Russell Hobby said the tables should not have been published and added: "This year we saw the SATs system descend into chaos and confusion. Delayed and obscure guidance, papers leaked online, mistakes in test papers and inconsistent moderation made this year unmanageable for school leaders, teachers, parents and pupils.
“The data gathered in primary assessment during 2016 is misleading. We warned the government that publishing this data in league tables could lead the public and parents to make poor judgements about a school’s performance, but it has still chosen to do so.”
The children who took the SATs in the summer were the first to be tested under the new National Curriculum, which came into effect in 2014. As such, they had received two years' education under the old curriculum, something West Sussex County Council said "impacted upon results".
A council spokesman said: "While we are obviously disappointed with these results, since the beginning of the autumn term we have been working very hard with our schools to support them in improving pupils’ performance."
She added: "In April 2016 we put in place a school improvement strategy and this includes a Key Stage improvement plan to support and challenge schools with improving their Key Stage 1 and 2 outcomes.
"This is designed to build confidence with schools in delivering the new national curriculum and secure improved preparation towards national assessment and moderation in the summer of 2017.
"Each of our West Sussex schools has a Linked Advisor and schools are supported according to their need, with those requiring the greatest improvement receiving the greatest level of support."
Make the right choice for you and your child - Visit educationsussex.com for trusted school reviews, videos and so much more.