The main curriculum of the third year was the casting of the Patronus charm, including the formation of the Patronus.
However, Dementors certainly could not be permanently stationed at the school, or the school might as well be renamed the Azkaban School of Witchcraft and Wizardry.
So Lord Voldemort planned to have Dementors stationed at Hogwarts every seven years, a year that would be known as the Year of the Dementor, striving to ensure that every generation of young witches and wizards could encounter a collective lesson on Expecto Patronum during their school years.
Chapter 217