Complex sentence: Until 1890, high school graduates prepared for the entrance examination to the ENS by repeating their "classe de rhétorique" (the current high school junior year) and teachers would give them more difficult assignments than to junior high school students.
Split into simpler sentences: Until 1890, high school graduates prepared for the entrance examination to the ENS by repeating their "classe de rhétorique" which corresponds to the current high school junior year. Teachers would give them more difficult assignments than to junior high school students.