Because we teach Cambridge exam preparation courses at my school, the issue of 'standard' English often arises. In this case, we distinguish between American and British English, but obviously, as other posts have pointed out, there are many Englishes spoken in the world today.
What I tell my students is to try to be consistent in spelling and pronunciation. Since the University of Cambridge recognizes American English as a 'standard' English, it can be used on the writing and speaking portions of its test. However, Cambridge does inform instructors that if a student spells 'colour' the British way, (s)he should also spell 'humour' with a 'u.' In addition, students shouldn't call the trunk of a car the 'boot' one time and a 'trunk' the next.
Since students are living in Southern California, listening to American news programs and meeting Americans, I advise them to focus on American English idioms, pronunciation, and spelling. For the Cambridge exam, they do need to recognize British accents, including Scottish and Irish, but they don't have to imitate the pronunciation themselves. They also ought to recognize that a 'fortnight' to a Brit is two weeks to an American and a 'queue' is a 'line.'
For more differences between British and American English expressions, click here. For a very extensive compilation of information on the subject, see Wikipedia.