Class, grade, rank, rate, graduate, gradate are synonyms in that they all involve the idea of ordering a number of things according to a scale or of placing a thing in its due order.
Class usually presupposes a division of a number of things of the same nature into groups; it implies, therefore, an arranging based upon some such principle as age, advancement, quality, or convenience, and a placing of items within one of the resulting groups.
What class presupposes, grade definitely implies as a fact or a possibility: grade also carries a clearer implication of division into groups based on an ascending or descending order (as of quality, merit, advancement, or social status).
Rank primarily means to arrange in rows or in any serial formation; it, like class, presupposes a division, but, more specifically than class, it implies a division based upon merit, proved worth, or social standing, and a placing of any one person or type or sometimes thing in its proper place or in order of precedence.
Rate (see also ESTIMATE, DESERVE) suggests a determination of the class, or grade, or rank and is used especially of ships and of seamen or ships’ officers or of motors or generators with respect to units of power.
Graduate like grade implies the existence of an ascending or descending order by which something may be classified or into which it may be fitted; the term, however, stresses rather the existence of differences than an actual division into classes.
Gradate carries further the trend of graduate and implies a changing or passing usually by imperceptible degrees in the direction of either an ascending or a descending scale. The verb is used especially of colors and is less common than the related noun gradation which is of far wider application.