Kate's family has class. I wouldn't put them in the same group as Fanny's parents. While they are not royalty, they seem to know how to move in higher circles. Kate is a quick study. She has been able to figure out the curriculum quite well. I think she knows how to take advice. I don't know anything about her changing schools just so she could bump into William, that may be true. In any case, it's worked out very well for the both of them. It certainly is not impossible to jump social and economic classes, but you need to follow the rules if you want it to stick. Fanny doesn't like to follow rules.
I forget who said or wrote that HRH Catherine's social ascent has been the result of, well, two things, 1) winning the love of a royal prince, and 2) more than two centuries of social climbing efforts by the Lupton and Middleton families which were mostly yeoman farmers, merchants, academics and clerics, that eventually afforded the Middleton kids a decent upper-middle-class life that opened doors of privilege. Some distant cousins married into the aristocracy, while others became politicians, but a majority were just hard-working individuals who eventually made better lives for their families. Catherine's modest background, charisma, intelligence, style, and humility make her a better mother and one day, a better queen.