An Essence of Cheating?

I just have finished marking my last class who had done the exchange rates worksheet and their marks make me feel suspicious.

Although they are the furthest behind in my teaching (due to timetabling), they scored the best overall, with only 6 out of 39 of the students getting below 60%, and 12 getting 100%.  I find this oddly suspicious as the lowest mark in the class was below 10%, which indicates that either that student is very poor and almost the whole of the rest of the class are  genii, or that most people in the class cheated.  I was not in the lesson that the worksheet was set, but in the lesson before, I handed out marked copies to the class next door (which had a much more believable spread of results)!

To try and catch out any cheaters, in the lesson tomorrow, I'm going to put some similar questions on the blackboard and if nobody can answer them easily call out some of the names of the students who got 100% and ask why they cannot do it when they managed it so easily on their worksheets.  I will tell you how this goes tomorrow!

Better be off (again) now - I've got over 45 stars to stick on the worksheets because, although I think they have probably cheated, I've got to stick to the reward system that I am using with my other classes else it would be unfair!