OK, so another nativity thread. Sorry.
Ds1 8 was given his first part this year as a wise man. We were happy as he has some self esteem issues, which his teacher spoke to us about at parents' evening (of course we are aware of them too) and so I thought it was really kind she was encouraging him with a part.
At some point (I became aware of it last weekend when I saw the script), he had an understudy - as did a couple of other parts. Anyway, he said, "I'd better be good because 'Kevin' is catching up with me fast". Then on Monday the teacher came up to me and said he said his lines in a 'weird voice' (he was losing his voice). On Tuesday his voice was nearly gone so 'Kevin' did the rehearsal and I was told they will be 'sharing the part'. Now, there is only 1 performance, so I clarified what that meant and teacher said they will be sharing rehearsals. After Tuesday (the only day he hasn't rehearsed for 2 weeks), ds1 did the part and took it in turns with Kevin.
Now Kevin has been given the part. Ds1 said to me, "I've failed. Kevin is being a wise man." 
Kevin may well do it better, but this is not acting school. My ds has performed before in church plays and he has been great, so I can't see that this one-liner would cause such huge problems. Of course I totally played it down, in fact I have been saying since I became aware of the understudy. So he does not seem unduly upset, but you never know with ds1, he is not good at expressing when he feels really hurt.
I am 
as she knows about the self esteem issues and I am angry she did not tell me herself today - so cowardly.
So, what do I do? I don't want to make ds1's life hard by complaining. She is the kind of woman you don't mess with. I also know it is the end of term, teacher's are stressed etc. Advice.