"I wouldn't want to accuse you of cheating but it could be seen as suspicious the fact that over 70% of your students have been given first class honours marks for your course."
"Firstly they weren't given the marks. They earned them. Secondly unlike many other courses I have concentrated on developing critical thinking and argument not just stuffed them full of facts. If you would like to interview any of my students you are welcome. Additionally as you can see from the external examiners report 'this methodology has produced stunning results.' I wouldn't want you to accuse me of cheating either.
If I had been cheating getting only 70% of my students 1st class marks would mean the cheating wasn't very successful would it?"