Curious as to what is a normal amount of accidents at preschool?

We moved my 2 1/2 year old DD to a Montessori preschool (along with our 4 year old DD) thus school year. I was just notified by the principal that my daughter, along with a handful of other students have been having too many accidents during the day. They are on a 2 week trial in hopes that they improve otherwise we will have to re-evaluate. I'm a little surprised given she hasn't been having accidents daily. It's been maybe once a week. The last week or so has been a little off due to a stomach bug causing very loose stool. But even then, it was only one day last week & then again yesterday. Potty training has been a work in progress for her. She will not be 100% accident free in two weeks.