What is the business scenario of having a default watched category?
If I have understood watching/following right if all classA parents get notified whenever a parent opens a new topic or replies, it will get as noisy as the old bulk cc emails.
You could instruct the classA representative (the parent who is elected as pivot interface with teachers and school administration) to open a new category-pinned topic whenever there is something official (e.g. class meeting schedule, class outings, etc)
I keep meeting the requirement for a strong PING from a specific member of the community (a parent representative, the director of a no-profit association…), just as it happened with the old email list. I will test the scenario I just described and report.