When asked if they see "great risk" in drinking that much, almost half the teens questioned -- 45 percent -- didn't see it as a big deal.
The study being released Wednesday by The Partnership at Drugfree.org also showed upward trends in marijuana and Ecstasy use among young people in grades 9 through 12.
"You're seeing this weakness in this generation of teens' attitudes around drug and alcohol use," says Steve Pasierb, president of the partnership. "It's not like this generation of kids thinks they're more bulletproof than others, but they really don't see any harm in that heavy drinking."
Among teens, the average age when they had their first drink was 14, the study said.
Overall, 68 percent said they had consumed alcohol. Of those, one quarter of teens had their first drink at age 12 or younger.
"It is much more terrifying these days than it was when we were younger," says actress and mother Melissa Gilbert, national spokeswoman for the partnership.
Gilbert, the former "Little House on the Prairie" star, is herself a recovering alcoholic who at one time was drinking two bottles of wine a night but has been sober now for six and a half years. She has faced drug-abuse struggles with one of her four sons, and she says she is doing everything in her power to make sure her youngest, now 15, knows the drama and pain addiction can bring.
"The most important thing is to get to know his friends and stay in constant communication with the people that are around him all day -- his teachers, his counselors at school," Gilbert told The Associated Press in an interview from her Los Angeles home.
Being sober, she says, is the best way to live. "It's not the easiest, but it's definitely the best," she said.
According to the study, teens said the top reasons for drinking were "because it is fun" and "so they won't feel left out."
Pasierb says early drinking can often signal deeper problems. "It's about that vulnerability," he said, "Why is a 12-year-old drinking?"
Gregg Aguero of Houston, Texas, says he started drinking regularly at 13 after his parents split up. That led to cocaine and other drugs and eventually landed him in rehab for several stays.
Now 22 and in college, Aguero says he's been sober for 4 months and is trying to help teenagers avoid the mistakes he made.
"It's never too late," he tells them. "That's the most important thing. It's never too late to turn and get help."
Other findings in the study:
--Twenty-five percent of teens said last year that they had smoked marijuana in the past month. While that number is unchanged from the previous year, it is higher than 2008 and confirms an upward trend that ended nearly a decade of declines in pot usage among teens.
--Ecstasy abuse also continued an upward trend, with six percent of teens reporting past-month use -- up from four percent in 2008.
The Partnership's "attitude tracking" study was sponsored by the MetLife Foundation. Researchers surveyed 2,544 teens with anonymous questionnaires that the youngsters filled out from March to June of last year. The study has a margin of error of plus or minus 3.6 percentage points.
Based in New York, The Partnership at Drugfree.org is formerly The Partnership for a Drug-Free America -- perhaps best known for the "this is your brain on drugs" ads of the 1980s and 1990s. The group launched its new name last October, a move meant to position the partnership as more of a resource to parents and to avoid the misperception the nonprofit is a government organization.