Many students seem to think that the following is true:
long sentences = complex = high score
This is not true. Long sentences are just short sentences that have been linked together. It's easy to write a long sentence. It's also easy to write a long sentence which is full of mistakes!
You don't need to be Shakespeare to get a band 7 (or band 9)! You just need to write clear, organised sentences which answer the question and contain some good vocabulary.