You can't please them all and you should not try.
Let it role, and stop thinking about it. If no consequences came out of it then treat it as over.
Now, if you want a little satisfaction get a copy of the signed letter. Slip it to the girl and quietly tell her you hope she will put the same effort into getting her work finished in class. And say it matter-of-factly.
I've done it and I figure it let's the child know two things. 1-You know what they did and it does not mean anything to you. and 2-you still care about the work she does and you expect more effort from her since she proved she can do somethng if she puts her mind to it.