Don't quote me on this as I haven't been taught report writing either. However I've been looking around online lately, studying reports as well as getting advice from family and I've written below what I think is a fair summary of report format for the exam.
For the exam, the nature of the report seems to warrant an informal report, which is described as a report "used for internal use, particularly within departments and for dealing with routine issues. Memorandum format is often used for these reports." (a short formal report seems to require too much information to be the right choice in the exam).
The report would start off with something like this:
MEMO TO: Board of Directors
FROM: Peter S. Roberts
DATE: May 24, 2006
SUBJECT: Proposal To Attract Younger Customers
Make sure the headings are in block capitals. They should also be bold, but you can't really do that with pen and paper so I wouldn't be too concerned about it. Instead, underline them

Next, have the following titles and paragraphs below:
IssueThis is the first paragraph, simply write the subject in more detail, developing it a little bit. Only needs a sentence or two.
RecommendationAnother quick sentence about what you think the board of directors should do, you will then state the recommendation again in more detail at the end of the memo.
BackgroundThen you would obviously, as the question will probably suggest, put across good and bad points for the proposal with any stats and figures (if you're fortunate to have come across some previously and remember it in the exam) and if possible construct graphs and charts out of them if you have time. If you do include charts and graphs, reference them in the memo as something like "Chart 1 shows there to be a 28% increase in profits..etc". If you don't it's fine, suggesting the advantages and disadvantages is the most important thing to do.
Then close with your thoughts on the findings, and make sure you don't introduce any new information in your conclusion, only refer to what you've already made note of in your findings. Finally, provide your recommendation again of what the company should do based on your findings. There's no need for a complimentary close as far as I know, I haven't seen any on professional reports. I've seen business reports use a reference initial to close, which are the initials of the inputter and sometimes the writer, but this will not be needed in the exam.
Number each paragraph in the margin as well. This is the basic gist of a report.