As you say, it is a puzzle what to do. I see two courses that might be of use: the first would be to appeal to the person who applied the curse, supposing you know who that was; the second would be to send the child to St Mungo's for intervention.

We are very near the end of term, as you know, and I feel certain that under the circumstances, you might ask Professor Slughorn to seek the Headmistress's consent for your daughter's early dismissal; I would be happy to make the necessary arrangements with St Mungo's.