According to a Piper Jaffray survey of high school students released on Tuesday, 6 percent already own an iPhone and 9 percent expect to buy one in the next six months. That’s twice as many teens as owned iPhones in Fall ’07, three months after the device was first released, when 3 percent had already bought one and 9 percent planned to.

Overall, Apple (AAPL) did well in the survey, which sampled 389 U.S. teenagers and showed the company’s lead rising in this key demographic.

iPod market share among the group was a record 86 percent, up from 82 percent last fall. And among the 39 percent who legally purchase music online, 81 percent said they used iTunes. That’s actually down some from the 89 percent who used iTunes a year earlier, but it’s not too shabby considering that a majority of the teenagers in the survey download their music from P2P services rather than paying for it legally.

Piper Jaffray analyst Gene Munster, the lead author of the report, surmises that Apple’s share of the legal download market may be falling among teens despite their strong preference for iPods partly because other online music stores are selling DRM-free music that is compatible with the iPod. We assume he’s talking about Amazon.

In any event, the survey shows that despite slowing sales for, say, fashion and footware, U.S. teenagers — or at least 6 percent of these 389 teens — still have money to spend at the Apple Store.