For expanding a short article that doesn't provide encyclopedic coverage (known as a stub), beginning from a sandbox can be helpful. Here you can write and rewrite before going "live."
Small articles that are expanded by a factor of five within a short period (and are well-referenced) are also eligible as "Did You Know" entries; working in a sandbox until reaching that threshold may be a good idea.
Your role as an Ambassador
Wikipedia Ambassadors who are familiar with the Did You Know process should help students nominate their articles and make sure they aren't nominating articles that fall well short of DYK standards.