When I completed voluntary work I found it best to try and talk to the teachers before the lesson starts, ask them what activities will take place and how you would be most helpful - they are always grateful for an extra set of hands! If you have any relevant skills or experience then make it known, but mainly just be prepared to get stuck in and be yourself! Lunchtimes are a great opportunity to talk to teachers, support staff and other trainee teachers - pick their brains, they will be able to give you honest advice and information! If you get the chance, make some notes on teaching styles, classroom management techniques etc as it will all come in useful for your personal statement/interview. Hope that helps!