‘I wrote the screenplay for Water Lilies while I was studying screenwriting at La Femis film school in Paris, and the director Xavier Beauvois, who was on the graduation committee, told me I had to make the film myself. My starting point was that I wanted to talk about girls, and in particular to portray the tough job of being a girl.

‘I wanted to play around with the codes of the teen movie. In Water Lilies you have three clearly identified characters: the chubby, funny Anne, the beautiful, blonde Floriane and the childish, awkward Marie. When you first see them, you think you know where the film is going. I wanted the audience to feel comfortable with this, so I could then take the viewers on a journey that they wouldn’t have predicted.

‘Usually in teen movies you have relationships between boys and girls, but here the boys are more like fantasy figures, who don’t speak. And there aren’t any grown-ups because I wanted people to have to identify with the female characters and to feel what it’s like to be in the mind of a 15-year-old girl.

‘The synchronised swimming sequences in the film are a hook to get people to think the film is going to be about that sport. To me synchronised swimming is a metaphor for the job of being a girl. They have to fake their smiles, they’re made up like dolls and they can’t show the effort they’re making, yet under the surface of the water it’s a struggle and a sacrifice. What you show and what you hide sums up what it is to be a teenage girl.

‘I filmed Water Lilies in Cergy-Pointoise, a middle-class suburb about 20 km outside Paris. It’s where I grew up and where Eric Rohmer filmed My Girlfriend’s Boyfriend in 1986. The area sprung up very quickly in the 1960s and it has very strange architecture. To me it’s very French, but at the same time the red brick housing estates could come from a small American town or they could be from somewhere in northern Europe.

‘I wanted the ending of Water Lilies to be open, so that everyone can have their point of view about what will happen to the girls. After screenings some people say to me that a particular character is going to be gay, or that she will go with a man. Above all I didn’t want the film to be classified as a gay movie and Marie to be seen as some sort of martyr. At that age every desire is possible. You have to deal with that and it’s difficult, and saying that makes the film political in the widest sense of the word.’

Water Lilies, Cameo, Edinburgh and selected release from Fri 14 Mar.