P.M.
Assuming your daycare has multiple kids to keep track of, I can understand they would be reluctant to face the possibility of multiple accidents during the day. A plan like this would best be discussed with the preschool ahead of time, instead of expecting them to "comply" with a plan that they had no say in determining.
I do know (now and in the past) at least a few families who proceeded with various potty-training strategies just on evenings and weekends. The kids seemed to do pretty well with it. As the new habits took hold, their motivation grew, and they were able to translate their successes to the daycare schedule.