Imagine being a 4h grade teacher with a class of 25 students with widely different levels of math aptitude and competency. How do you challenge the most adept students? How do you help the laggards without boring the stars to tears? The only practical option seems to be to teach to the middle. That it, unless you and your principal are lucky enough to have discovered Kahn Academy, which is in the process of disrupting the long-staid textbook publishers market and K-12 education.  Kahn Academy is a stellar example of how any business today can thrive by capitalizing on open-market trends that are melting barriers to entry in most markets. Kahn Academy was started by a Salman Kahn, a hedge fund manager and brilliant communicator, an educator with a passion for teaching math and other lessons to kids. He developed effective on-line math…