The DfE has recently published non-statutory guidance on gender separation in mixed schools here.
This guidance, which applies to publicly funded and independent schools, follows the Court of Appeal’s judgment in the Al-Hijrah School case and will support schools on what is expected of mixed schools. Schools should not generally separate pupils by reference to protected characteristics such as sex, race or faith while at school. Any separation by reference to a protected characteristic is likely to give rise to unlawful discrimination unless permitted by the specified exceptions in the Equality Act 2010. Details are set out in the guidance.
Any school that continues to separate pupils against the guidance is in breach of the law, and this will be reported where these schools are inspected.