Most of children can follow rules in front of a teacher. So, I think the best way to solve the problem - to be in the same room with children most of time and constantly remind them about the rule. When they learn this rule very well a teacher can be out of the room for some period of time. There is one more way to avoid the mess. You can take ALL toys away. The kids will have nothing to make a mess. But I do not think that this is a good idea.
