Hi Woolsy84 my birth dd is 10 and my adopted ds is 4 now, so we are in a very similar place to you. We have decided that whether to tell people about adoption is our son's choice, and so I try not to tell people about it if I can avoid it, unless it is in his best interests. So of course I will tell doctors and teachers etc if I feel they need to know. Like when a new doctor asked if ds had reacted to injections before, I had to say I did not know.
Having a dd already at the school means most people already at the school are probably aware ds is adopted. However, increasingly I think people will just forget. So I do not feel the need to tell them.
Before ds went to preschool (at a different location) I told him if he wanted to talk about birth mum etc he could but then people would know I was not his birth mum. And it would be easier if he talked about birth mum with me and dh, and dd, and did not talk about it at school. Not because it was bad or anything but because it was private for our family. Likewise with dd before ds came I told her she could talk about it to her best friends but not to the class in general.
As time has gone on, and reading views here, it has appeared that often adopted children do not want to appear different to others, so if I tell all and sundry they will know, they may know anyway because I am quite well known in my small area and having a dd at the school makes it likely all those connected to her and me will know, but those at the lower end of the school will not.
So it is a personal choice how much you give away information wise, allowing for the fact that some people will just know.
I totally agree about strategies for children to keep things private.
Of course he is special because we chose him but to me dd is no less special because she came about after a series of fertility treatments and investigations. And likewise my close friends know this and so does dd but I don't feel the need to tell everyone else.
I think bullies really can pick on kids for all kinds of reasons, a friend of mine was bullied for being slim, others I have heard of are bullied for being fat. I think the solution is to build as much self confidence into the children as you can and also to totally nurture a talking and sharing environment where any problems at school are discussed and you can intervene if and when you need to.
I hope both your little boy, and mine, and all of ours will not experience any bullying or teasing for any reason and hope school will go smoothly.
Just a heads up one friend told my dd that ds was not her brother as he had not come from me!
DD is working out her own thoughts on all this
so you may find you need to engage with your dd about all too, and good luck with it all.