The expression sécher les cours literally translates as ‘to dry classes’ and means to not attend lessons, to skive school.
It is said to date back to when inkwells were embedded into school tables and so when pupils did not attend class, the ink would be unused and ‘dry up’, inspiring the expression.
Another expression to mean playing truant or bunking off is to aller à l'école bussionnière, which translates literally as ‘to go to school in a bush’ It has its origins in the sixteenth century when Lutheran priests had difficulty preaching in public due to Catholic opposition and resorted to building clandestine schools in surrounding forests and fields to share their teachings. So if you were there you were not in your (usual) school.
Equally, the expression faire le mur - ‘to make or do the wall’ - means to leave a place without permission and is often, although not exclusively, used in the context of playing truant, especially with boarding schools and sneaking out at night. It is an evolution of the much older expression sauter le mur, meaning ‘to jump the wall’. The phrase is also used when speaking of prison escapes or soldiers leaving their barracks against orders.