I teach French in elementary school, and have been using the scoreboard successfully since the beginning of the school year.From what I understood at a conference with Chris Biffle, it was better to keep the incentives "small" and progressively increase the "rewards" along the year (if needed). 1 minute extra (or less) recess could later be increased to 2, 3 minutes..I always include in my lesson plan some engaging, fun activities for the children, and save ONE as the incentive (a quick game, a French riddle ,a mini craft, 2-3 mn coloring, pointing to the alphabet with my laser pen, ...). I am a huge fan of WBT.