If you are saying that they are three black children, within a very small minority of black children in the school and they are fostered so the school have a legal requirement to protect them I'm assuming you mean by protecting their identities because they are fostered, and you are saying that them being in the minority makes them more identifiable. If that isn't what you are saying then I can't see the point of mentioning either point.
If that IS what you are saying then they shouldn't have been in the picture at all (although if that was the case surely that would be the cause for concern not a jumper) and as their foster carer you also have a responsibility to make sure the school know not to include them, and that the children know too.
If your point is about being identifiable then changing a different jumper to one that matches will stop her standing out.
But as your op didn't mention them not being in the picture for legal reasons due to foster care, and you were complaining that her individuality was taken away by making her look like others, I don't see how you can later say that it's because the school have a legal obligation and the children stand out because of their skin colour when your issue originally was that they STOPPED her standing out by changing her jumper.
I get the feeling you didn't like the responses you initially got, so completely changed your angle, thus potentially contradicting yourself.
Make your mind up op - did you want her to stand out in her short sleeved jumper, or worry that she stood out too much because she is black in a predominantly white school? You can't have it both ways.