When I play around with individual stocks, I have used finance.yahoo.com. They have a lot of junk news stories in their feeds, but summarize all of the basic info in an easy-to-use format.
Once they pick a few companies they want to learn more about, each company's investor relations website is a great place to learn more about what they do and how they make their money. Depending on how old they are the annual reports may be really interesting, or overwhelming.
My problem with the school stock market game is that it's set up to encourage super-short-term gambling style trading, so I might encourage them to use it as a chance to learn how different kinds of businesses work, and de-emphasize the "game" part... But that might be tough.