Quoting Mama Rice:" My DD (she is 5) had an accident at school. She peed herself. It is very few and far between she has ... [snip!] ... abnormal or bad she had an accident and she is in kindergarten or should I just chalk it up to a one time thing and not worry?"
Lily had 2 accidents in kindergarten. She never went to preschool or anything and wasnt used to "We are doing activities, you need to plan to pee" kind of thing. So she was playing and oops... had an accident. It happens, and it's nothing to worry about.