As of 2008, there were 16,375,000 high school students in the United States (source). There are roughly 200 school days a year. Using your value, that means that roughly 664,800 students will drop out in a year. That's 4.06% of the high school population. This government website pegs the figure at 8.1% (as of 2009). It would seem then that the value in the commercial is an underestimation. Using the values I've provided, it's closer to one student every 13 seconds (if I've done the math right).