I find the soft sell works better. Teenagers are super-judgmental people, and running around with a puppet can look like the height of dorky and uncool. I remember back when I was one of the founding members of my college improv troupe, recruiting people involved hanging out with them, saying "sh*t, you're really funny. I would love to get you on stage with us." and then hoping they might take us up on that.
To me, puppetry is a combination of a few things. It's all about stagecraft and movement. It's also about technical work, special effects, and theater magic. With improv, we approached all manner of other disciplines; people interested in comedy, theater, public-speaking skills, writing. Seek out your dancers, your actors, and your stage techs. Half of my college troupe's members were not performers; we had stage managers, treasurers, lighting and sound directors, and campus liaisons. Someone in your school wants to be involved in staging a show, even if it's behind the scenes. Find those people and your group will grow.