Complex sentence: At that time both teachers and students wanted Italy to go to war, but when the Great War started, the school had a difficult and terrible period caused by economic problems for teachers and the fear of fighting for lots of students.
Split into simpler sentences: At that time both teachers and students wanted Italy to go to war. But when the Great War started, the school had a difficult and terrible period caused by economic problems for teachers and the fear of fighting for lots of students.