One plays many games in a short time at a tournament and this compacts the benefit of a month's practice into one weekend. Also one sees other teams who do the same thing and have to play the same challenges. Watching how they rise to the challenges of questions will also be very instructive. Watching their errors can also be very instructive. One will learn how to cut losses on difficult bonuses, get a better idea when to avoid taking -5s, learn to allow one's teammates to answer questions in their specialties, etc.
Players get a lot better by sticking around.
It takes some enthusiasm, but also patience. Everyone improves with practice. Those high schoolers who compete in quizbowl and then play when in college certainly have a leg up on other unexperienced college freshmen. Even if you can not run it this year, their interest in helping out may lead them to join your club in college. Also rememeber high school tournaments are a possible source of revenue.