Murmurs of excitement rippled through the large hotel conference room when the eight special guest panelists filed in. Empty chairs filled and doorways crowded with onlookers. Cell-phone chats ended abruptly. Spectators leaned forward, eager to catch every word. These were no ordinary panelists to the officials of clothing companies, food giants, media dot-coms and others attending the marketing conference. They were American teen-agers, articulating the consumer habits of a group projected to spend a staggering $155 billion this year. So they applauded the answers of these Chicago-area high school students, laughed at every attempted joke and flooded them with questions for an hour. Where do you buy things?