Complex sentence: During the first years of schooling the number of pupils per class doesn't exceed 22, and later on (Upper Prep and Senior School) they are divided into various sets depending on their abilities and electives being taken.
Split into simpler sentences: During the first years of schooling the number of pupils per class does not exceed 22. Later on (Upper Prep and Senior School) they are divided into various sets depending on their abilities and electives being taken.