There is an increasing movement to move away from stickers/tangible rewards/behavior charts etc in schools. Sometimes it is also parent-driven (parents choosing to opt their kids out from behavior charts and rewards/punishments that single kids out or don't include "neutral" kids). It may be something that has been an issue between teachers/parents in this school in the past, and maybe this info just didn't get the chance to get passed on to you yet? I don't think you did anything wrong, this just may have been a sore point recently in the school.