Chat about school in normal everyday conversation but keep it light and positive.
Accept , validate and normalise their feelings about school. It can be especially difficult after a school holiday or sickness, eg. “Your right, it can be nerve wracking going back to school after a break. I bet there are lots of children who feel the same way.”
Plan some fun and interesting things to do in the evenings and weekends to give them something to look forward to and remind them that school is only a part of their week.
Try and have regular family feedback time which makes it normal for everyone to share their worries from the day as well as the fun things that happened. You can role model this by telling your child about things which have happened for you, eg “Two people in the office are leaving this week and I feel sad about this.”
Teach them a simple breathing technique and let them know how useful you find this yourself.