Can ED-breaking upperclassmen hurt your chances at admission?

Agreed. And I don’t know whether colleges do or don’t inform the GC. What I do know, though, is I’ve been here a long time, as have you, and the vast majority of people who worry about the HS being blackballed as simply clutching at straws. I think, and it’s only my opinion, that a HS has to exhibit some really egregious behavior to warrant blackballing. Every GC I know is too professional to not counsel the students/parents on the ED agreement and the ramifications of breaking it. The reputation of the HS, and their own reputation, is at stake.

I’ll also say that many of these types of posts start with “I heard…” “someone told me…” etc which brings to mind a quote from SpongeBob, “Why, once I met this guy who knew this guy who knew this guy who knew this guy who knew this guy who knew this guy who knew this guy who knew this guy who knew this guy who knew this guy who knew this guy who knew this guy who knew this guy who knew this guy who knew this guy who knew this guy who knew this guy’s cousin…”
https://www.youtube.com/watch?v=FEVaXyu0lWc

Or, from Ferris Bueller’s Day Off;: “My best friend’s sister’s boyfriend’s brother’s girlfriend heard from this guy who knows this kid who’s going with the girl who saw Ferris pass out at 31 Flavors last night. I guess it’s pretty serious.”
https://www.youtube.com/watch?v=MLWGDxGh2G4

Or, from REO Speedwagon’s “Take It On The Run:” “Heard it from a friend who / Heard it from a friend who / Heard it from another you been messin’ around”
https://www.youtube.com/watch?v=3G8jAPPjpGs