two items I have used to demonstrate magical principles are the cut and restored rope and the colouring book:-
the cut n restored rope is used as an example of
selling a trick were the secret has already been done before the trick starts, i.e. the performance is the trick not the secret work unlike sleight of hand, this is why you should not underestimate a self working trick I'm assuming your not giving away secrets to curry favour in this demonstration.
the colouring book is used to great effect as a demonstration of misdirection, blank, outline and coloured works best, show the colour pictures and then divide the pupils into two half's i.e. left and right, you say to the right side you can only see the outlines as you are now temporarily colour blind.... the left side you are totally blind.
ok all pupils on the left look at the pictures now place your hands on your head.... ok now you can't see anything the pages appear blank..... ok put your hands down and your back in the room... show pictures
repeat same with right side and show outlines, the whole audience will see the changes but you explain the putting of there hand on there head is the misdirection that allows you too swap the book before there eyes.
this idea also plays well with a change bag and printed silks, in that case all of the pupils place there hands on there head, hope this makes some sense as its not a full scripted answer.
feel free to contact me if I can be of any help.