I don't know if this will work but ......... Turn it into a challenge! Tell the class that there will be a competition and that your aim is to make them talk. They have to ignore you and get on with whatever they're doing (getting changed quickly) without speaking. If you manage to get anyone to speak, you win. If no child speaks - they win. Say that you will be walking round the classroom trying to 'trick' them into talking. You might for example, ask them a question or tell them to say their 6 times table. Usually they need to do as they are told but this is a fun 'game' where they are not to 'give in'. Maybe you could give a point each week (to yourself or the class) and reward the winner at the end of each half-term. I've used it very successfully with younger children. It's worth a try.