During the written part of the exams, you'd have to keep your scientific calculator near you at all times. Some students would discretely smash calculators of people looking in the wrong direction.

Frankly I hated prépa. The worst time of my life. But I do recognize that it teaches you a sense of organisation.

Another problem with prépas is that you work so hard to get into a Grande Ecole, that when you make it there your work motivation goes down (you feel like you've reached an El Dorado of sorts). In University the system is smarter ... you work harder as you go along, each step of the way.