When you start to revise larger chunks of an existing article, it's a good idea to draft your first significant edits into a sandbox. Don't copy the whole article, just choose the small piece you want to work with.
Trying to rewrite an entire article in a sandbox can be annoying to editors who work on that article. Other editors will keep making good edits or expansions to that article while it's in your sandbox, which you'll kick out if you just copy and paste! Focus on small chunks of the text you want to edit instead.
It's polite to use the talk page of the article to link to your sandbox when you copy things over. That way editors can see what you're up to, and post suggestions to the talk page before you start. Once you are happy with your sandbox draft, you can place another notice on the talk page of the article with a link to the sandbox, explaining what you've done and asking for comments on it once you've added it to the main page.
Your role as the expert
The sandbox stage is a good opportunity for you, as their instructor, to highlight major problems and point students in the right direction to fix them. Make sure students are using high-quality sources, and make sure they are rewriting information in their own words rather than copying the sources or committing plagiarism through close paraphrasing.