One morning, instead of turning up for class wearing the regulation navy blue smock, a defiant group of adolescent girls came to school in white T-shirts instead, demanding an "end to discrimination".
At the elite Bizerte public school in the north, as is the case in most high schools in the North African country, pupils have to sign a school rule stipulating that wearing a uniform applies to girls only.
Ironically, the warning was passed on during a philosophy class -- about the human body.
This "injustice" inspired many of the girls to take to social networks and vent their feelings, 18-year-old Siwar Tebourbi told AFP.
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She said the girls agreed to take collective action from the following day "to demand that this discrimination must cease".
So dozens duly turned up for class, wearing white. Several boys did the same, in solidarity with their classmates.
It was the culmination of a dispute that had been brewing for years.
Outraged that the navy blue was imposed on everyone in primary and secondary school but was compulsory in high school only for girls, pupils regularly appeared without it, risking expulsion or seeing their parents summoned.
Monia Ben Jemia, head of the Association of Democratic Women of Tunisia, an independent feminist group, called the smock rule "a terrible message" because it implies that young girls' bodies can have a disruptive effect on their peers.
The high school students who launched the campaign, both male and female, are also against what they perceive as a wider "hypocrisy".
"They drill into us at school that men and women are equal, but in practice this is not the case," said Adam Garci, 17.
That the navy gilet is actually supposed to erase social inequalities between pupils is a source of some amusement to Tebourbi.
"If it was really meant to conceal any differences between rich and poor, then boys as well as girls would have to wear it," she smiled.
"One supervisor told me I couldn't wear leggings without a smock because I was 'shapely', and another told us 'It bothers the men teachers'," Ben Jemaa said.
The whole affair would appear to be somewhat embarrassing for the authorities.
Speaking on condition of anonymity, one senior education official found it difficult to explain exactly what was happening.
He acknowledged the sensitivity of the subject, even though Tunisia is considered to be a pioneer in North Africa and the Middle East in the field of women's rights.
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