Complex sentence: Its popularity meant it had to develop a rigorous selection process so by the mid- 20th century-the school already being considered an elite institution-admitted its students after two years of dedicated preparatory classes through a competitive entrance exam.
Split into simpler sentences: Its popularity meant it had to develop a rigorous selection process. By the mid- 20th century, the school was considered an elite institution and admitted its students after two years of dedicated preparatory classes through a competitive entrance exam.