She maybe holding it when at school, and then just has to go so badly that it "leaks" even without the urge to go poop.
Talk to her Teacher, and make sure she is getting adequate potty breaks and TIME in the potty. I know at my Son's school the K class only gets 1 scheduled potty break mid-morning, My son started having accidents because he needed to go for a sit down potty break just prior to closing activities, but he couldn't express that well enough. He'd say I have to go Potty, and his teacher assistant would just take him expecting only pee, so he didn't really have enough time to poo.