Preparation for meeting the HoD will depend on the individual student's situation.
Students who attempted to warn the teacher of their non-attendance should document their efforts to do so.
Students who really could not attend should document the road closures and transit cancellations that prevented them from attending.
Minors who were told to stay home by their parents should get a note from the parents taking responsibility and explaining their reasons for keeping the student home.
Anyone who did not attend without a good reason needs to think about the wording of an appropriate apology.
Students who could not attend and made at least some effort to notify the teacher are in the strongest position. Even those should be apologetic about the group non-attendance if they encouraged it.
At the other extreme, I don't see anything other than an apology for a student who could have attended, chose not to, and made no attempt to notify the teacher.