The head referred to your family as what?! Who exactly was that too? If your daughter I'd have been on the phone to the LEA!
YANBU, your daughter made a genuine mistake...she behaved responsibly, did the right thing...and then, very bravely owned up to the mistake...the embarrassment for her, would have, in my mind, been enough! Agree with others that your XP needs to call, confirm the mistake was his, and end the issue...Do be prepared to hear the schools side though...I'd like to think that there is a degree of misunderstanding here.