Complex sentence: He entered Yale as an undergraduate in 1921, as the twenties began to roar, and found an invigorating academic environment, but also a social stratification that kept the students from more privileged backgrounds apart from the others.
Split into simpler sentences: He entered Yale as an undergraduate in 1921, just as the twenties were beginning to roar. He found there an invigorating academic environment, but also a social stratification that kept the students from more privileged backgrounds apart from the others.