@evantill your question is right, you can bulk invite via csv to a topic, but you can’t autowatch something.
Still, the parents will only access their own class category and optional school-wide categories, each member will have to develop her own habits and preferences regarding notifications.
But this situation is not optimal for important urgent messages (e.g.“Tomorrow’s trip to the museum canceled, bring kids to school directly.”).
EDIT: instead of compulsorily watching a category, one could think of group-wide @-like mentions. There was I think an obscure syntax to address a group by means of an #groupname syntax, but when I tested it it did not work.