Why did expanding secondary education make teacher preparation more difficult in the 19th century?

The expansion of secondary education in the 19th century complicated teacher preparation primarily by creating a tension between a teacher's personal need for a broad education and their professional need for specific classroom techniques.1

As human knowledge became more diverse and organized into new disciplinary systems, the simple, inclusive pedagogical methods of the past—such as the monitorial system—fell into disfavor. Educators faced the difficult task of reconciling the requirement for academic subject mastery with the demand for practical training in school management and pedagogy.1

Furthermore, teacher preparation was influenced by various social and class-based considerations, such as the desire for practicality in the United States or the rigid class structures in England, which often limited the intellectual scope of elementary teacher training to only the subject knowledge required at the school level. This shift necessitated a move toward more systematic training, eventually leading to the integration of general and professional education in colleges and universities, as the previous reliance on limited, practical-only training became insufficient for the evolving needs of schools and pupils.12