GCSE will often be the first time students are expected to write full-length essay-style answers to questions. This can be a daunting prospect for many students and so I think the best way to start would be to give them a fairly rigid structure which they can then move away from once they gain confidence.
The standard length of a GCSE essay is 1000 words - and I would suggest this should be spilt into roughly 5 paragraphs of 200 words each.
The first would be an introduction in which a student would briefly directly answer the question and outline the main points of their essay.
I would then ask the student to write topic sentences to begin each of their three paragraphs with in order to make sure they are clear and focussed in their response to the question. I would suggest the student use a P.E.E (point, evidence, explain) structure for each of their points, so as to ensure they back-up their points with both quotations and analysis.
The final paragraph would then be a conclusion in which the student would sum up their main points again, reflecting the introduction, but without adding any new information.