According to the memories of the Dark Lord and Severus, Harry Potter was able to cast a full Patronus by his third year, and other students could manage it by their fourth or fifth year at the absolute latest. Therefore, for students in the fourth year and above, the final exam requirement was to successfully summon a complete Patronus.
Of course, Slytherin and Ravenclaw students were treated differently.
Those were things deeply rooted in their inner natures, very difficult to alter; one could hardly force them to change their innate dispositions, right?
Chapter 223