I suppose it depends on your contract. Are you on daily supply or are you covering for a maternity for instance? If it is daily supply I would expect the school to provide you with some 'busy' work for the classes. If you are on a longer term contract then I would expect you to be planning lessons for your timetable. As fot the twiliht meetings then I would expect the school to add the hours to your timesheet so that the agency can charge them for your time and pay you.