Rishi Sunak has promised schools will receive guidance on policies for transgender pupils for the summer term.
The Prime Minister intervened after a teaching union said its members were navigating a 'minefield' around gender.
He was asked about a report from a centre-right think tank Policy Exchange which looked at 154 English secondary schools.
It found only 39 of them reliably informed parents when pupils identified as trans or questioned their gender,
Eighty-seven schools found parents were not reliably informed, 14 did not provide enough detail and a further 14 chose not to respond.
The authors of the report, written as part of the think tank's Biology Matters Project, submitted a Freedom of Information (FOI) request to 304 of England's 24,000 secondary schools.