Complex sentence: Plato was the pupil of Socrates, and the teacher of Aristotle for over 20 years before Aristotle left to found his own school, the Lyceum, and had been an aspiring dramatist before coming under Socrates' tutelage.
Split into simpler sentences: Plato was the pupil of Socrates, and the teacher of Aristotle for over 20 years before Aristotle left to found his own school, the Lyceum. Plato had been an aspiring dramatist before coming under Socrates' tutelage.