When I was in high school, I would go to an open mike night about once a month at a local cafe / tea shop. The place was standing room only, with about half of the people still in high school. I don't know if it would kill the ambiance of your tea shop, but you may want to consider having an acoustic open mike night once a week or every other week.
Now, the teens are obviously less likely to order tea, but if you advertise a cheap special of a very flavorful tea at the counter or invite them to try a sample with any order, they may be more likely to get hooked.
Just an idea