Think about something that stresses you out. Perhaps it is going to a social event, having to do a presentation at work, or even the simple act of leaving the house. Think about the amount of time you spend thinking about what may happen when you are faced with this stressor. What if it all goes wrong? What if someone sees you go through this and fail?
Now imagine that you are a child and that stressor is school. For many children, the start of school can represent the prospect of prolonged, continuous exposure to stress.