Secondly (more controversially), is a picture of a noose racist? I mean, it certainly has racist connotations and I personally wouldn’t have used it, but (bear with me) I’m not sure racist is the concrete conclusion. Lots of people have been hung throughout history, if you’re not viewing it through a racial lens then are you a racist or just very insensitive.
I see what you're saying... but it seems like a technicality that's not worth exploring here.
If instead it was a guillotine with a caption of "this year's detention activity"... it's really not any better. Like it's no longer racist but... now it's just purely about killing kids in a more equitable homicidal format. It's very dark humor in a very public place.
It's a lot of time and effort and argument to debate "yeah this was terrible but it might not have been racist". Does a definitive answer of "at least it wasn't racist" make this appreciably any better? Does it make them any more likely to get a teaching job after this?
It also doesn't change the core argument the teacher is making in the story. "You lumped me in with a bunch of racists because I was white. I took down the image because it was offensive. Obviously. Why would I leave it up if it was offensive?"