Focus on the positive by asking your child about what things about school they're excited about, and let 'recess' or 'coming home' count as answers. Reassure your child that he or she will be safe. And as school begins, be sure to praise your child for being brave and facing their fears. And yes, this even works for teens too, if you approach the conversation in the right way.
Watch what you say, though. There's a difference between being cheerful and unintentionally adding worries to the mix. General reassurances such as, "Everything will be fine!" aren't solution-oriented. And statements like "This is a big year for you," or "This year really counts" are stressful and set unrealistic expectations. Try to redirect the worries to the positives.