A council is facing legal action after a group alleged updated guidance on how transgender children should be treated is unlawful.
Under Oxfordshire County Council policy, boys can share toilets with girls if they identify with that gender and vice versa.
Opponents say the approach is "in direct opposition" to safeguarding law.
But the council said it "utterly refutes" suggestions it was putting children at risk.
The action is being brought by a parent, Victoria Edwards, who lives in Oxfordshire, another parent on behalf of their 13-year-old daughter, and a teacher.
Ms Edwards said: "By creating a cast of people to whom safeguarding does not apply, the local authority is leaving the door open for abusers to walk straight through into the lives of our children."
Safe Schools Alliance UK (SSA UK) is supporting the legal action over the council's Trans Inclusion Toolkit for Schools 2019.
It said the toolkit must be withdrawn or the council would face a judicial review.
But the council said updated guidance had "led to some improvements" in how trans pupils are treated at schools and colleges.
A spokesman said: "While we acknowledge this is a difficult and emotive area, we are confident the revised toolkit will provide helpful guidance to schools looking to support this potentially vulnerable group of young people."
Paul Conrathe, a human rights solicitor who is representing the claimants, said: "Central to its legal error is its bold assertion that 'trans girls are girls and trans boys are boys'.
"This guidance has whole-heartedly embraced transgender ideology at the expense of the rights of others. It is surprising and concerning that a public authority could issue such a document."