班

班 (bān) is a high-frequency noun with three connected senses. First, 'class' — a group of students who study together: 我们班有三十个学生 ('our class has 30 students'). Second, 'work shift' — used most often in the compounds 上班 (go to work) and 下班 (get off work): 我每天九点上班. Third, a 'scheduled run' of transport — 班机 ('scheduled flight'), 加班 ('extra train / extra shift').

The same character covers all three because the underlying idea is a fixed, scheduled group or assignment. English speakers should note that 班 is rarely used alone in conversation; it's almost always part of a compound. Counting classes uses 班 itself as a number-suffix: 一班 ('Class 1'), 三班 ('Class 3') — common on school doors and timetables.