My teen volunteered with these folks for three years before heading off to college.
The gowns are gorgeous and there's thousands of them. Teens also get one free accessory to go with their free gown like a purse, shoes or set of earrings or a wrap. Just bring your valid student ID card. We get girls who drive from California and Washington just to get a free gown. The only drawback... there's quite a line. You may need to "camp out" a bit to get your dress. New venue for passing out dress this year is the Oregon Convention center.
You can also donate gently used gowns to the cause. It's a great event.