My suggestion is not to show them code at all, to me the code is just the language we learn to solve problems. Like learning to speak another language, it doesn't change your message, just how you deliver it.
So instead I would present them with a few unique problems that Computer Science helps solve. Whether it is a web site problem, an Arduino or Rasberry Pi based small robot or something large that will catch their attention.
I think showing them how solving problems not only involves critical thinking but also creativity, a no-barriers thought process (e.g. no preconceived ideas on solving an issue), and how you can really impact the world with it. At least in my opinion and experience, kids like to be inspired and don't think in the terms of boundaries so showing them how Computer Science really lets us do that could leave a great impression.
I'd also take a number of slides and have a picture on each one of different things that Computer Science helps solve, e.g. Manufacturing Automation, Welding, Cars, Space travel, websites, communications etc. Make sure at least 2-3 are things they would have never understood were backed by programming, I think that impacts people.
I don't know how french high-schoolers may differ from the US, but my bet is they share most qualities. My son was inspired very much in this way when he was in middle school, and he jumped at the chance to join a technical program for high school.
Good luck and have fun, that is an awesome opportunity to help shape kids view of the world.