Possible duplicate of:
Using contracted forms (“don't”, “let's”) in a formal text
Usage of contractions like “it's” and “that's” in textbooks
Should contractions be avoided in formal emails?
In Germany, our English teachers always taught us that contractions like "didn't", "he's", "won't" and so on are perfectly valid to use also in written English. The only difference to writing the long version, consisting of two words, was that it would only count as one word.
However, in Singapore, where I currently am for an exchange year, if you use these contractions in a test, they will be marked as a mistake. The teacher says it is not allowed to use them in written English.
So my question is: Are the German or the Singaporean teachers right? Is there any official rule on this? How is this marked in other countries' schools?