Ah brings back flashbacks to my swimming lessons. I remember how terrified I was. Gosh if you really went through with it that would have brought you quite close to sith in my own meter hehe.
The other way you could do it is to start the class with a pop quiz announced the previous time. That would certainly do the trick. Might be too cruel though haha.
I suppose you can be cruel to be kind? Being too kind and they take you for granted sometimes. It's a fine line that we thread.