Liquid asks a good question. My son started peeing in his pants (sometimes even pooping) after his brother was born. My husband is an accountant and when busy season sets in, he alwasy has an accident or two as the family adjusts to the new schedule.
I also agree that it might be something she can control, so she uses it to feel secure. If she had a negative experience at school or on the potty once, she may be fighting against having to face that thing again. Take time to think about any changes, major or subtle, in her routine at school, at home, at daycare, or in her bathroom experiences that may have affected her. Once she talks about it and can verbalize (even a little) what may be bothering her, the issue may stop on its own.
Is there a new addition to the family? I think mayby she may be looking for some sort of attention. Try telling her you will put her back in diapers if it keeps up. And if you tell her that, and she continues, follow through with the diapers. It will resolve the problem because that is what she wants, or it will shame her into controling her bladder. But don't enforce it with anger. Enforce it with regret. Its something you have to do because you said you would do it. Only you know the cercumstances involved. This is only a suggestion. It sounds to me like she only pees enough to soak her underwear but not her pants. That sounds like control to me. I could be wrong. If you think it is control, see previous suggestion.
could it be a thick bladder wall? Some children can't feel it until their bladder is so full it leaks. I know this seems like a big deal to you right now- but she will learn to go at intervals at school time and will improve in time. Tell her to go empty her bladder even if she feels there is nothing in there at school times and every 3 hours at home during the day.....