A simple activity which could last a few lessons..depending on what you/the students find.
Possible extra apparatus you could use: mirrors, cotton wool buds to help counting (don't use cocktail sticks unless you have a few hors spare to fill in the incident forms).
The clear picture on the worksheet shows students which teeth are which and all they have to do is count their own and record the numbers. They can try on their own in pairs, use their tongue to keep track or cotton wool buds to "tap" the teeth as they count.
A mirror is very useful.
The activity usyually lends itself to much discussion regarding missing teeth, extra teeth, decay, bad breath, bacteria, diet etc.
I like it because you can just watch the students find out about "themselves" remind them it is not a competition, unless you want it to be.