Complex sentence: Southern Illinois University started offering a bookkeeping class in 1894 then gradually added other business courses but it wasn't until 1957 when a separate School of Business and Administration was established under the leadership of the school's first dean, Henry J. Rehn.
Split into simpler sentences: Southern Illinois University started offering a bookkeeping class in 1894 then gradually added other business courses. In 1957 a separate School of Business and Administration was established under the leadership of the school's first dean, Henry J. Rehn.