Someone who exudes so much class (i.e. maturity, intelligence, poise, worldliness, some stereotypical components of being a WASP, etc.) that he/she is immediately respected and noticed for his/her class. This level of class is also almost completely natural and effortless. A classmaster's class is constantly pointed out--it is that noticeable.
May also be used sarcastically to mock someone doing something totally not classy.
Maggie: Did you know Elizabeth's parents have been feeding her pâté since she was around 5 years old? She's also fluent in French.
Andrea: Yeah, and she wore a pencil skirt and blazer to school today. What a classmaster.
Katie: I'm gonna wear my sneakers with my dress because I don't feel like wearing my flats.
Rebecca: OHEMGEE, you're SUCH a classmaster!