In the message, the principal of the Gymnasium Oberaargau, Barbara Kunz, said it had come to the school's attention that “in the warmer weather, views were being granted that would best be avoided.”
The principal recommended the girls cover up more, for their “own protection and out of respect for others”.
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A graphic accompanying the email suggested girls avoid strapless tops, and “permanently visible” underwear.
“Remember that you are not going out or to the pool,” Kunz said in her emailed message.
But girls at the school have reacted angrily to the recommendations which makes no mention of boys clothing.
The graphic used by the Oberaargau school in its email was copied from that used by a German high school. But the original image also included guidelines for male students, which the Swiss school had edited out.
The original email from the school to students.
“We are being treated as objects. This is clearly sexist,” one 17-year-old female student told Swiss news portal 20 Minuten.
“If school directors are going to make recommendations, they should do it for boys and girls,” 18-year-old student Michelle Stauffer told the news site.
Meanwhile, students have put up English-language signs in school bathrooms reading: “Instead of shaming girls for their bodies, teach boys that girls are not sexual objects.”
“Our thighs, legs, shoulders, bra straps or stomachs are not distracting. They are simply a body part,” the A4 signs also state.
Students also told 20 minutes that some of their classmates were now dressing provocatively for the first time to spite the principal.
But Kunz on Monday defended the clothing recommendations telling regional daily Berner Zeitung that the intention had been to draw girls' attention to the impression they might give.
She said the clothing issue regularly came up in warmer weather and that her message did not mention either controls or punishment.
The school principal also noted the image included with her email was sprinkled with humour while her deputy Robert Zemp said school uniforms were not the goal and that the idea was to help boys and girls to deal with the freedom they enjoyed.