Especially William, he had to complete all his studies before he came of age, then spend a few years in the Military, and only after that could he proceed to University for further education.
Politics, history, psychology, economy, management, culture, language, mathematics, and so on were subjects William had to understand and learn.
In addition, including etiquette of the Nobility, horsemanship, shooting, public speaking, and debate, composed of William's traditional noble curriculum.
Of course, that was still far from over. Before he came of age, he would also engage in sports, along with some musical instruments and painting among others, only then would his education be considered complete.