I wonder if it could be a bladder infection, I know you say she has no discomfort. Does she do the same at school ,I know sometimes children do not like to ask to use the bathroom at school could there be any problems there ? Maybe allow her to go to the bathroom when she wants and not tell her to ,it may make her feel bad .
I agree with Margy, many times the fault kind of lies with the schools due to the kids fearing to use the bathrooms or teachers not allowing them to go unless its recess time (and thus a madhouse in the bathrooms).
However, that does not explain the weekends. But I think giving her a bit more freedom on the weekend to learn how to deal with this is also a good idea.
And, of course, its always worth checking out any medical reasons. This is a bit unusual at this age, so looking outside the box is always a good idea.
Also, I have seen studies that link bathroom problems to AD/HD and actually its pretty common with kids that have ADHD. But I would think that either you or her teachers would have noticed the symptoms of AD/HD? Hope this helps.
I would ask the pediatrician his opinion.
Margy, the fact that she can pee on demand suggests there is no medical problem. And she obviously has bladder control if she does not wet the bed at night. Could her behavior be something as simple as spite or stubbornness?
But she doesn't seem to miss not spending time with her friends, and that is odd.