faire le mur

to slip out of somewhere without permission

Used for boarding-school pupils or off-duty soldiers who climb over the wall for a night out they're not supposed to have.

Les internes ont fait le mur pour aller voir un concert en ville.

The boarders snuck out to go see a concert in town.

Goes back to the early 20th century, when it took over from the older phrase 'sauter le mur' (to jump the wall), simply swapping 'jump' for 'make'. It stuck especially in military barracks and boarding schools, where the wall in question was a very literal one.