A contact list would, like what likely existed for every previous reunion in GPs high school since before 2010. My schools periodically reach out to update your contact info for their alumni roster. I bet GP never gave the school their contact info after graduation rather than this being a facebook only situation (unless it was just organized by a handful of alums and not tied with the school itself).

There isn't a passive tool that would solve this other than getting people's email or mailing address, and that's absolutely fine since if you wanted to go to these reunions and be invested in these events, you can send an email to the school and get yourself on this contact list.