Flora G found your mark scheme interesting.
How about marks off for :
-lots of repetition-spinning out the song boringly or
-for singing the same songs week after week or
-for a ridiculous number of verses (one song I timed recently on my phone was over 11 minutes long and everyone was looking mighty fed up.)
-for talking through a performance
Dont mind people using sheets as a prop but can't bear really badly prepared stuff (i.e. not prepared)
Can anyone think of any more?