Start a conversation with your child by saying that you have been thinking about the new school year. State any facts you know about the school (new or old), teachers and classmates. Ask your child what they are looking forward to or concerned about. Discuss all of it. If they don't mention adoption, ask what they will say if a peer or teacher asks.
Create a generic teaching moment:
Lots of children are adopted every year.
Say they won't answer (It's private) or Pretend they didn't hear it: