the problme sfx is that most concerned professionals who dont have an intimate knowledge of adoption tend to treat in a very very jolly "ooh isn't it interesting everybody" way or use it as an opportunity to educate everyone on how different everyone is When in fact everyone isn't different in this way.
And its really not fair to do that when they are 5 and aren't even that aware of their difference - most children don't even begin to process what their adoption means until they are 7 or 8 and certainly don't need it pointed out to the whole class. Its a bit like bed-wetting! Not something to be ashamed of when you're a child but still not something you'd necessarily feel comfortable having a group discussion about.
I just to make sure that teacher understands that I will be coaching DS in a way that avoids the issue without lying - "I was born in Kaz and moved to England with my mummy when I was one" kinda thing. I just don't want it hijcked by well-meaning outsiders! DS is quite welcome to discuss what he wants with whom he wants when he is competent to make an informed decision.