Accused of slander by school?

School head has been shown a private message I sent to another parent via her facebook account inbox. The print off given to head should state "between you and (other parents name)". In this message I wrote "I have big issues with (head's name) elaborating on events". This is due to phone calls received from her stating serious issues only to go into school to discover it's not as bad as made out or to be told by another school employee, ie, deputy head, that the head had exaggerated. Head rang me to say legal advice was being sought for slander and I would be sued. I believ it's actually libel if anything! My quote was my belief and as a result of my experience and was made in private, not via my public facebook wall. Does the head have a case against me?