More than 1,000 private school children in Calderdale could be forced to go to state school if Labour follows through on plans to abolish private education.
The Labour Party has pledged to end the "tax privileges enjoyed by private schools", after members voted at its annual conference to absorb all pupils and wealth from the sector into the state system.
A January census undertaken by the Department for Education shows there were 1,229 pupils. That's three per cent of pupils in the area.
Some may travel from homes outside of Calderdale to attend their school of choice.
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Private School Policy Reform, a new independent think tank, launched a report in September outlining options for reforming the independent sector.
These ranged from scrapping private schools’ charitable status and charging tax on student fees, through to nationalisation.
But the Independent Schools Council said a move to abolish private schools would constitute "an attack on the rights and freedoms of parents to make choices over the education of their children".
It added that the party's decision was "an ideological distraction" from dealing with the real problems faced by state schools.
Chief executive Julie Robinson said: "Every family with school-aged children would be affected if independent schools were abolished.
"State school class sizes would swell further, resources would be stretched and the financial strain on already-stretched budgets would be enormous."
One in six secondary school pupils and 11 per cent of primary school kids in Calderdale are taught in classes that are too large – those with more than 30 pupils.
Some classes have even swollen to 36 or more – 210 primary pupils are in classes of this size.
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Labour's Angela Rayner, shadow education secretary, said: "Boris Johnson has broken his promise to tackle the tax privileges enjoyed by private schools.
“The next Labour government will immediately close those tax loopholes and use that money to improve the lives of all children.
"Our Social Justice Commission will look at how private schools can best be integrated into the education system to make it fairer for all children, regardless of their background.”
Across England's state schools, the number of pupils in large classes has increased in recent years.
In January, 11 per cent of primary school children were in classes with over 30 pupils – up from 10 per cent a decade ago.
For secondary schools, the figure is nine per cent, up from seven per cent in 2009.
A Department for Education spokeswoman said it would continue to ensure parents have a choice in where to send their children.
She said: “The UK boasts a diverse education system, in which state schools, independent schools and universities are encouraged to share their expertise and resources through our partnerships programme.
“The gap between state funded schools and independent schools has never been smaller – 85 per cent of state-funded schools are now rated good or outstanding compared to 68 per cent in 2010."