"The OP said her child's class were in a position to overhear the fact that her daughter had a counselling session. She didn't say the class had been told why she had the counselling session."
It hardly matters. They know she attended a counselling appointment - more detail than necessary/comfortable.
" The information that someone has a medical or counselling appointment alone - no details given of the reason - is - according to the training I have received - not considered specific enough to count as a data breach under GDPR regulations."
You might want to check the small print. Is telling everyone whether it is medical or dental or counselling fine? I suspect not. I suspect it isn't mentioned at all.
"I have clearly stated that if the class had been told the REASON for the counselling appointment, there may be a breach of GDPR involved that may be worth pursuing."
I should bloody think so. You still fail to recognise the problem, though.
" I have not said anything here that is factually inaccurate according to the training I have received."
Indeed. Because there's no predetermined right or wrong. But you've failed to grasp the nettle. And that's either because you don't understand the training, or because it was shite.
"I really don't understand why so many people are taking umbrage at this. I am just stating how the GDPR regulations are interpreted when it comes to what is classified as sensitive data in a school setting. Whether you like it or agree with it or not, those are the facts."
Dear me. We have tribunals for this reason. How GDPR regs are interpreted is not up to you or anyone who trained/misinformed you.
It's law. You follow it, best you can. But in the event that another party is unhappy, they can question it and, ultimately, take you to court. It is then for the judge to decide.
You are quoting your personal understanding of your employer's policy as though that is binding for all.
I think that OP would absolutely have a case based on the "counselling" part of their complaint. You disagree. Nowhere in UK law can you show me that I'm wrong.
Stop saying this is absolute.