Letter to the form teacher of head of year
Dear Ms Smith,
When I saw you yesterday and explained how upset my daughter Mary was due to bullying by Jane Brown, you said you would (do some work with the class / monitor the situation / speak to Jane's parents / keep me informed).
Mary knows that she must tell you if she has any further problems. Can we please have a meeting in a couple of weeks to review the situation?
Yours sincerely,
<Your Name>