Back when i played that game in my school, i came in first by investing in penny stocks. I used the ticker pnsn, which is about 1-2 cents a share and goes up a cent about once every day or so and goes back down very quickly, so you have to watch it. This may not work very well depending on how accurate your game is because that company has a really low market cap, so you can only buy a certain amount of shares, which i think was like around 100,000 or something. At my school, the game we used had no market caps and you could just buy however much you wanted, so at one point i was buying over 10,000,000 worth of shares at a time. After a week or so of doing this, i didn't even need to play anymore to win. So if you have no market caps, use that stock and you will destroy the other kids and your teacher will think you are a god.