Most of us are more comfortable in any given situation if we know what to expect. Kids are no different. Talk positively to your kids about school -- and learning in general -- and have detailed conversations about what the school day might be like. Have a little fun with the scenario if it helps.
Most importantly, open a dialogue with your kids about what makes them feel anxious about going back to school. Maybe it's something obvious, such as attending a new school or going to a school with older kids, but the reason(s) may surprise you. If you don't ask, you can't help.