Some girls are missing school because they feel so uncomfortable using gender-neutral toilets, the Mail on Sunday reports.
Gender-neutral toilets are becoming increasingly common across schools in the UK as a way to be inclusive towards children who do not identify with their biological sex.
But the newspaper reports that parents and teaching staff are concerned about some girls are avoiding using the toilets because they feel unsafe or uncomfortable about sharing them with boys while they are menstruating.
Some are refusing to drink all day in order to avoid needing to use the toilets while at school, the newspaper reports.
Parents at Deanesfield Primary School in South Ruislip, West London, launched a petition last month opposing the introduction of unisex toilets.
A mother of two daughters at the school told the Mail on Sunday: "The cubicles were open at the bottom and top so older pupils can easily climb up the toilets and peer over."
GP Tessa Katz said girls who avoid going to the toilet for long periods of time are at increased risk of urinary and bladder infections.
"The psychological effects of girls not feeling safe enough to use mixed-sex toilets is also concerning," she said.
The Scottish Government recently withdrew transgender guidance for schools after a backlash from women's groups and SNP members.
The guidance had recommended that schools allow transgender students to use the bathroom and changing facilities of the gender they identify with.
Controversially, it also stated that parents need not be informed if their child expressed a desire to change genders.
The document recommended that if parents or other children raised objections about the policy, they should be told that the school was committed to inclusion.
"If parents of other children voice concerns about their child sharing toilets and/or changing rooms with a trans learner, they should be reminded of the school's ethos of inclusion, equality and respect," the guidance said.