Be Prepared!
And I do mean ANYTHING!!!
Preschoolers can change in a millisecond. Happy one minute, angry and throwing a fit the next.
They wait till the last minute to go to the bathroom, thus accidents happen.
Your perfect child in class normally may just be having a bad day.
All of these things, and more, and normal for preschoolers and you have to prepare for anything. Parents are good to help (or tell you how the day has been), but this is YOUR class and you need to know what to expect and set your expectations accordingly!