There are two approaches for this direct and indirect; you can try whichever resonates with you.
Direct: Especially if your child is used to having heart-to-hearts with you, take a moment to bring up the topic of back-to-school. Don’t do it while your child is in the middle of acting out, wait until you are in an easy space – maybe driving, or during family dinner. Sometimes kids can’t connect their behavior to their emotional reactions, so they might not first be able to link the two together. Mention what you’ve been noticing and question where it might be linked to going back to school.
Indirect: If your child isn’t used to heart-to-hearts like this, rather than put a spot light on them, find some time to casually start a conversation about back-to-school. You may talk about how you felt when it was back-to-school time mentioning the mix of excitement and anxiety. Then see if you can find out how your child is feeling about back-to-school too. Just starting the conversation will let your child know that you are paying attention and that you want to help.