[Anonymous]

Hi Paul, 

Welcome to the Support Community and thank you for posting this question. 
I would encourage all other teachers out there to share their way of working with you in regards to this, as they might have some useful tips for you. 
Have you tried switching to design mode and locking the objects/illustration?  Once you are back in presentation mode, these objects cannot be changed. 

You can right-click the objects and lock them from the sub-menu that comes up. 

Your reference:  00464357