I think that's a little unfair; and it's different from country to country.
In Germany, for example, if you used an American spelling in a mainly British English text, it would be underlined and marked "AE" (for "American English") in the margin, but you would not lose any points for it. And if you used a British spelling in a mainly American English text, it would be marked "
BrE", but again, you would not lose points.
"Meant" is the correct past tense of "mean" in both the US and Britain. However, the past tense of "spell" is "spelt" in Britain and "spelled" in the States. Again, it's probably better to use "spelled" as it is also acceptable in Britain.
From what I understand, American and British spellings are both acceptable in Canada, although you would need to be consistent.
In Australia, the decision whether to use American or British spelling can be a political statement. British spellings are usually used by more conservative, traditionalist writers, American spelling by more progressive or left-wing writers. This can be confusing: many people write "labour" the British way, but generally May 1st is called "Labor Day" (American spelling). Wikipedia manages to spell it "Labor Day" when referring to the US holiday but (strictly incorrectly) "Labour Day" when referring to the Australian version
on the same page; but
an official Australian government website uses the American spelling. The mainstream opposition party in Australia is officially
The Australian Labor Party, but
on at least one page they use the British spelling to talk about "labour" as a synonym for "work".
It confuses me, too.