"One would hope that his education background might have given him some insight into what "slope" was, and how to best explain it to someone in the 8th/9th grad

"One would hope that his education background might have given him some insight into what "slope" was, and how to best explain it to someone in the 8th/9th grade." That leap you made there at the end of your sentence is the crux of the issue. Most educators argue that being an expert in a subject is not even remotely sufficient qualification to teach it effectively, especially to children.