S.L.
It sounds like she is, IS, potty trained at school. So the issue is who is in control at home. I think she's testing you and she knows at school she just does it. My advice is to talk with the teacher and see how she handles it, does she tell the teacher every time, does the teacher tell them it's potty time, does she just go on her own, whatever you can find out. Then I would tell her how proud you are of her going at school and praise her and maybe even get her a SMALL little gift for that accomplishment. Then tell her she is to do the same thing at home and the end result would be a SMALL little gift and how proud you are that she can and will do it at home too. Then enforce it. If you have to start taking her every 15 min. all day on a weekend or when you're home then do that. If she doesn't go get up and go in 15 min. When she sees you mean business she will go as she is already trained at school. You need to be in charge on issues like this and sounds like she thinks she's in charge at home. Hope it works quickly because it should.