I had a daycare for 7 years in my home and not once did I have a child that repeatedly had set backs such as this. The only thing that concerns me is in all my training they say when a child is "stressed" potty training seems to be the first thing to go. Not to alarm you, but are you sure things are good at school? Are you sure they are taking her to the potty regularly? The only reason I ask is because if things are back to normal at school then she should be going to the potty. Maybe since she's in a pull up she's not a priority to take to the potty for the school staff? If's she's not having set backs at night or at home I'm convinced things are not right. Maybe nothing to worry about per say, but it's definitely worth checking into it.
In my day care I would have set backs all the time. But within a few days (if all was well at home) kiddos would get right back on track. Most of my set backs were actually at nap time which is never their fault. In fact, if a child showed up in a pull up I would immediately take them to the bathroom and put underwear on them. It's almost impossible for children to potty train in pull ups.
I understand the point of the school but it's not serving to her to make her wear a pull up, it's just too easy to forget!