Ask her all your questions, very clearly and bluntly.
Ask her what she really meant. Tell her why your response is so angry, disbelieving, puzzled.
Ask her to clarify the points that annoyed you.
Let her talk, listen and ask all the follow up questions that spring to mind, again bluntly.
Then repeat back to her what you understand, ask if you now understand what she meant.
Be polite, but leave her very clear why the letter made you so angry. Why it surprised you.
Let her tell you how she will put it right.
We have 'those parents' and angry parents in regularly. Angry parents usually have a point and, when they have got over the initial difficulty of how to talk to us we can sort something out that makes everyone more happy. When 'those parents' come in we go into a different mode altogether. More of a bunker mentality, wait for the moment we can pass them up the food chain, give them the address of the board, hide no contact detail and duck.
So by all means be angry, but don't be stubbornly correct and deaf to whatever she says.
Good luck.