That's the problem here. The disclosure as represented by the author themself is decidedly not responsible.
If you were in the same position as the author, how would you handle disclosure?
(Note: I shortened your quote, and the shortened bit does not give your intent. But it's the part I want to respond to. Other readers should read OPs full text.) The drivers of change with regards to the Equifax leak was the voting populous of the United States- I image if the Equifax hacker sent the credit reports of everyone compromised by the leak to everyone compromised by the leak and claimed to be an honest security researcher, it wouldn't have gone well.
> If you were in the same position as the author, how would you handle disclosure?
The author mentioned having the contact information of the legal council of the school. Send an email to them saying that email addresses are PII and they shouldn't be widely distributed. Escalate from there. If they ignore it, send a spreadsheet of names, email addresses, and facebook accounts to the lawyer. If they ignore that, start digging, send more - anyway, the point is, start at DEFCON 4 and work your way up, don't immediately step to DEFCON 2. In this case, where the solution is to paste into the BCC box instead of the TO or CC box, a 30 day window ought to be sufficient, but a zero day window is completely unacceptable.
Concern about being removed from an email list in a timely fashion?
"PII is defined as information: (i) that directly identifies an individual (e.g., name, address, social security number or other identifying number or code, telephone number, email address"
School groups for my kids actually share that information as a way for parents to contact each other.
I'm not certain that we can say that any given email list is required to be private.
Not using BCC on group threads like the one mentioned is essentially a PII breach. The fact that this was an organization responsible for the care of minors, which refused to remove someone who was erroneously given access to that data, definitely warrants concern.
If the author had found a security vulnerability in a school website, responsibly disclosed it, and deleted all information gathered, I doubt they would be getting the same level of side eye.