Teachers have a great deal of power over children in terms of rewards and privileges. But parents may ask teachers to do something that may not be in the best interest of the child, for example, keep the child awake all day so he will go to sleep soon after getting home. The child is cranky and out of sorts by the middle of the day.
Child comes wearing clothes that are inappropriate for playing in water and sand.
Parent wants child to learn alphabet and to read and write using conventional methods rather than inventive spelling. Or to use a computer. With young children the boundary between the teacher and the parent is often less clear as the teacher is responsible for many of the same things that the parent does. Children may tell you information that is usually not shared with persons outside the family. It is tempting to tell others.