It’s a well known fact that iTunes is popular among the teen market. But a new report from Piper Jaffray says that iTunes is now essentially the only option teens consider. Forty percent of high school students in the investment research firm’s bi-annual teen survey legally purchased music online. Of those, 97 percent said they used iTunes, up from 81 percent a year ago. Piper Jaffray attributes the growth in large part to slowing momentum from newer competitors, like Rhapsody, eMusic and Amazon (NSDQ: AMZN) MP3.