E.C.
how are potty breaks handled at school - are all the kids expected to go at certain times or do they have to let the teacher know when they have to go? If potty breaks are scheduled, maybe try telling her that certain times are "potty times" (could even have a little chart that lists the potty times) and that if she really needs to go between scheduled "potty times" she can tell you.
What helped with my girls was the "two-minute sit". A friend told me that she would have her daughter (who's about 2 years older than my DD's so my DD's looked up to her) sit on the potty for a couple minutes before they left the house (if she needed to go, it'd happen within the 2 minutes and if she didn't need to go during the two-minute sit, my friend was confident they could make it to their destination without an urgent "I HAVE TO GO POTTY!" cry from her daughter). So instead of asking my girls if they needed to go potty (because it seems that all small children answer "No" to this question), I'd tell my girls it was "time to do a 2-minute sit like Big Girl Sarah" and they'd willingly cooperate. Probably one minute would work too.