When I was a student, I spent my first 2 years in a so-called prépa intégré of a French engineering school. 20% of students failed and were shown the door during those two years (some failed, some figured that it just wasn't for them). That's fine, that means you keep the ones who actually do the work. At a certain point, you have to start treating students like adult, either they succeed or they don't but it's their personal responsibility.