This article is an addendum to On the Scent of Tom Riddle's Buried Bones, a quick read that will give some little foundation to this story.
On a subsequent trip to Edinburgh I agreed to take some on site photo's of a Harry Potter Funko with Riddle's grave and the school as backdrop. During this two girls approached me, I guess I looked like I knew what I was doing. They asked if I knew where McGonagall's grave was, debunking the "I know what I'm doing" theory. I'd had no idea that there was a second Potter-relevant grave site! But a little googling fixed that, and I did redeem myself by pointing out George Heriot's School, a nice additional site which they did not know about. This is a nice shiney one! Here's what you need to know. The plaque is on the south wall - the wall to your left as you walk through the Flodden wall arch. Look for the school, if you are at Riddle's grave it will be behind you and all the way to the right. Move to the south west corner of the kirkyard, you'll see a gate that leads to the school. As the arrow indicates your objective is on the wall next to the gate. In fact it is the last marker on the south wall before you get to the gate. It's a little like McGonagall is the keeper of the entrance to the school!