Operating a pre-school is probably one of the hardest jobs I can ever think of. They already say that parenting one child is already one heck of a job. What more if you are going to take care of dozens and dozens of children?
Well, sure, children are lovable. They hardly know any of the evils of the world, and they will most likely follow what you instruct them to do (except for some special cases, of course), but they can be really energetic as well--so energetic that you might lose all of your energies yourself running after them.
Pre-school field trips are not any better. These trips are usually organized to give the children a breather and a new environment that they will learn to cope with. This can be a nightmare for the pre-school teacher if the children decide to exhibit their most energetic forms in public at the same time. The children might get lost, or they might break something, or they might end up in kiddie fights and hurt each other. You can always stick something like a wristpass wristband with, say, GPS receivers to track them in case they suddenly get lost, but that can do only a little to take care of them.
In spite of that, a lot of pre-school teachers still love their jobs. Why, you ask? It's because they have the patience and, most of all, the passion to deal with kids, however unruly they sometimes can be. This is all part of growing up, and a dedicated teacher knows how to make sure that they are guided well.