Complex sentence: At the dawn of the twentieth century, writing slates were still the primary tool in the classroom for students and it was not until the 1930s (or later) that writing slates started being replaced by more modern methods.
Split into simpler sentences: At the dawn of the twentieth century, writing slates were the primary tool in the classroom for students. In the 1930s (or later) writing slates began to be replaced by more modern methods.