It's completely normal for a potty trained child to still have some accidents. And sometimes it might not be her fault. I work at a daycare, and there have been times that we just weren't able to get the child to the bathroom in time (maybe we were outside on the playground or just waking up from nap or taking care of another problem with another child). If it varies by teacher, though... that makes me want to know how many teacher transitions her classroom is having. Shouldn't she always have the same teacher? If there are frequent care provider transitions, that could affect her potty behavior.
Anyway, I am really impressed that your 18 month old is doing so well with potty training... most 18 month olds aren't that far along, yet. I would avoid putting her in pull ups, since you know she can be successful with panties... you don't want to step backwards. At this point, I would try to figure out a pattern. Which caregivers is she successful with and which ones does she have the most accidents with. Then go from there. I wouldn't worry about doing anything to alter her behavior (rewards, etc). And remember not to punish her for accidents because that can definitely cause a step backwards, especially if it's an issue with the teacher and not with her.
If it is an issue with the teacher, and the director can't solve the problem, then take a good look at your daycare and make sure that you really are satisfied with how they are meeting your daughter's needs.