Let's cut to the chase - language is not a fixed or static being, regardless of what protectionist institutions like the Academie Française or Íslensk Málstöð may intend. English, in particular, does not have any specific regulatory body. The OED is seen as the definitive say on linguistic matters, but this is only informally acknowledged and there are many others with valid claims to that title - Noah Webster's dictionary of American English was published at a similar time to Dr. Johnson's first dictionary of the English Language, so American orthography is equally valid.
Language change is a vital part of language history, and without it we would all still be speaking Proto-Indo-European (assuming that all of you have an Indo-European language as your first language, which is a massive assumption). Some of it is borne out of error or mutilation of the grammatical form (how many times do you see people write 'could of', 'should of', 'would of'? You never see that from anybody who's learnt English, only from somebody who has it as their first language), which can be irritating. But semantic change (like 'class' in your example) is part of language. Hence why the same word for 'horse rider' has come to mean 'nobleman' in English, but be decreased in meaning to 'stable boy' and then to 'servant' in German (Knight/Knecht). If Americans don't have a 'class' system that matches up to that in Britain, then the word in that context becomes redundant and is free to wander in meaning to something else, and thus it is being used in a completely correct and acceptable manner.
(Linguistics postgrad)