I think it's worth a chat with the Dr and having a UTI ruled out, they are really common in little girls and make it difficult to hold on to urine, as well as needing to wee frequently.
Other suggestions as to why DD is wet are:
The toilets are smelly and she hates using them, again lots of children would rather wet or hold on desperately instead. And quite a few will not poo there, holding on until they get home.
An encounter with another child or group has put her off. More of a problem where loos are shared with older children.
Lack of privacy.
She is too shy to ask to go, especially in front of a whole group. (eg during a story)
Too tired by the end of the school day, and especially when the change from one activity to another offers no opportunity to just get up and go.
Too engrossed in activities, and doesn't want her friends to know she is going to the loo, it's almost a sign of weakness, and a real nuisance when you're enjoying yourself.
My guess is that she is afraid you will be angry, which is why she doesn't tell you. Does she try to change herself when she gets home and hide wet pants?
Would it help if DD had set times to go to the loo during the day, to minimise the length of time holding on before she wets? What about on arrival at school, before playtime, before and after lunch, another convenient time during the afternoon if they don't have another playtime, and on going home if the journey is a long one. I know that it is best if a child empties their bladder when it is full rather than when it may only be half full, but if it helps DD to stay dry in the short term it could be useful. Then when she's dry all the time she can go to the loo less often, when her bladder signals full. Also, if you're not already doing so, send some dry changes of clothes along to school. It won't be forever.
I think that knowing you are not angry, and are trying to find ways of helping DD to overcome this difficulty will help her enormously. She must sense it's getting you down even if you don't say so, and I bet she feels ashamed about it. However, I reckon there are several more kids in her class who are regular wetters, so don't be too hard on her.