We dress differently when we go out with friends to how we would dress for an important interview. Similarly we change our language to suit the context, if we want to get the job!
Standard English is formally and grammatically correct English. It is the written equivalent of wearing a smart suit.
You are expected to use Standard English in all your English exams. This means not using slang, informal, chatty language, and making sure such things as your verbs are in agreement.
The only part of your English exam where you can use non-standard English is in the descriptive writing. But it pays to be cautious.
Only use non-standard forms when you are writing direct speech. That way the examiner can see that you're consciously adapting language to create specific effects.