When people get band 5 or below, it is often because they don't finish their essays, they go off-topic, they have no idea about good essay structure, or they try to use 'difficult' language and therefore make lots of mistakes. For bands 5 to 6, keep your essays simple and clear.
Many writers with the potential to excel fall short in their essays because of a lack of good structure. This is usually as a result of insufficient practice and gets worked out after a few years or even months if the writer is very diligent. For writers who would like to skip the confusion from the start, here are some characteristics to consider. A good essay structure must be:
Clarity within an essay also requires a good essay structure
First, a good written assessment should be just that—a good piece of writing. Unless you have specifically told students otherwise, it should represent the students' best efforts at spelling, grammar, and punctuation. It should have a strong thesis statement if that is warranted, and it should have a good essay structure, or at least follow the structure that you have delineated for them.