You know when children are asked to bring in 50p or a £1 or the likes for wearing mufti/dressing up etc and the class teacher may collect the money in? What do you think of a teacher asking the children in his class to pay some extra money towards it (very small amount) because some children took part in the activity but didn't actually contribute any money towards it? So the teacher tells the class that they each need to pay a small portion of it and he will have to make up the difference.