Not such a good choice
Articles that are "not such a good choice" usually involve factors such as a lack of appropriate research material, highly controversial topics that may be well developed, broad subjects or topics with little notability.
Trying to improve articles on very broad topics (e.g. Law)
Trying to improve articles on topics that are highly controversial (e.g. Global Warming, Abortion, Scientology, etc.) Your students may be more successful starting a sub-article on the topic instead.
Working on articles that are already of high quality on Wikipedia
Working on something only sparsely covered by literature. Wikipedia articles cite secondary literature sources, so it is important that students have enough sources to provide a neutral point of view and be verifiable.
Starting articles with titles that imply an essay-like approach (e.g. The Effects That The Recent Sub-Prime Mortgage Crisis has had on the US and Global Economics). These type of titles, and most likely the content too, may not be appropriate for an encyclopedia.