Moulton School in Northampton has merged two blocks to become a unisex toilet in an effort to cut bullying in 'intimidating' single-sex toilets.
But parents said the move could have the opposite effect - and teenagers with 'raging hormones' may shift their amorous liaisons away from the bike sheds and onto school property.
Mother-of-two Jayne Poole, 39, said: "If the kids are dating there will be no more 'round the back of the bike sheds' - it will be the toilets they escape to.
"At a time of raging hormones and teenage angst they need their own personal and separate space. I think this is a very bad idea."
However, headteacher Trevor Jones said: "We want to provide excellent facilities for our students and this design is intended to reduce both bullying and vandalism, as traditional toilet areas can be intimidating and unpleasant areas for students to go into."
The mixed toilet, which has two boys' and two girls' cubicles with a shared wash basin and hand dryer, has no mirrors, supposedly to stop it becoming a social hub.
One parent, who did not want to be named, said: "Girls, especially teenagers, care about their appearance, even more so when they are at a school with boys, so to remove the mirrors is ridiculous.
"They should be allowed to keep their appearance up, even if it is a simple thing such as making sure they look presentable.
"Even for the boys, they might want to re-arrange their tie if it has gone a bit wonky."
She added: "I have a daughter who goes to the school and she deserves privacy when she goes to the bathroom. Teenage girls go through changes, everyone knows that, it can be a delicate and embarrassing moment in their lives. It is just a gross breach of privacy."
The school, graded good by Ofsted this year, has more than 1,500 pupils and was made a specialist science college in 2002.
Headteacher Mr Jones said: "The new area is being used regularly and safely, and the only adverse comment that has come from the students is regarding the absence of mirrors; a deliberate decision to avoid toilet areas becoming areas for social gathering."