I would expect that you would have been told.

If your DD was in my class and I knew you were at the school somewhere I would have tried to seek you out to talk to you about it in person as opposed to writing it in a diary (mainly so I can answer any questions straight away as things aren't always 100% clear in writing). If I couldn't see you in person I would have made a phone call for the same reasons I just mentioned.

As an aside, I'd also question the confidentiality side of things. Did the teacher actually say to this parent that it was your DD this happened to or did her son tell her? I would never mention other people involved in a particular incident. I'd say that your child and another pulled a girls pants down but not mention any names.....