After the baccalaureate, the prep classes prepare you for two years for competitive examinations for the Grandes Ecoles: engineering, commerce, literature, etc. 800 preparatory classes in France welcome around 80,000 new students each academic year in public or private high schools, but also in establishments not under contract with the State.
The conditions are above all to be motivated, hardworking and to have correct results. Be careful, prep courses are not reserved for cracks, but it is better to only have above average grades.