I've done Ensimag, which is a public school in the same sector, and you definitely could skip classes if you wanted to (though most of the ones who skipped classes did it out of sloppiness)
Those who had issues were those who needed to be told where and what exactly to look for. They often felt like they were paying for little to no teaching (which is true, in a sense, but I preferred to see it as an opportunity to grow by myself in an environment where I could have extra resources at my disposal if I needed to - the best one often being other students, sometimes from previous years).
And once again, all engineering schools considers their students adult enough to let them do their thing: there's no repercussion if you don't go to classes, it's not 42-specific