Governors at the Towers School in Ashford, Kent, which has 1,400 pupils, have backed the uni-sex idea because they believe it will cut down on vandalism.
The move will mean that girls as young as 11 will share the bathroom facilities with 16-year-old boys.
Designs for two shared toilet areas have been posted on a school noticeboard, one with 11 cubicles for girls, 10 for boys and one for disabled pupils, the other with eight male toilets and 10 female. Both have wash basins in the middle of the room.
A sign underneath the designs reads: "These are just to give an idea of the proposed layout for the new unisex toilets. The colours of the cubicles and hand wash area we propose to have will be much brighter and the whole area more modern looking."
Parents are up in arms at the proposals and have been joined in their condemnation by ChildLine founder, Ms Rantzen.
She told the Mail: "These children are at an age when they are extremely self-conscious and aware of their bodies and the changes they experience.
"It's an extremely delicate time for them and one would hope that a school would seek to make them as comfortable as possible. This is one of the worst ideas I've ever heard and I suggest the school rethinks its proposal."
Dad Daniel Gray, 39, added: "Many parents are upset that the school has dreamt up these proposals without consulting them. Some girls will be as young as 11 and experiencing puberty – not something to be shared with teenage boys as old as 16.
"I think it is a bit much to ask teenagers to be responsible enough to give each other privacy. Girls in particular need privacy and there will be boys calling at them over the cubicle doors.
"Teenagers are full of hormones and I'm sure this will encourage them to canoodle in the loos – whether it's heavy petting or worse."
School governor Simon Petts said the governing body has approved the idea and is waiting for it to go to a funding committee.
He added: "The toilets have been in desperate need of refurbishment for a long time and we decided the best way of reducing vandalism was to make them unisex. I understand parents' concerns but we have had someone researching child welfare and this has not raised any red flags."
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