Our daughter's very independent and can struggle to transition activities (which I don't see as a particularly important skill, but most schools make a big deal out of it). 

The tailored nature of Montessori has always appealed, but it's especially good for kids like her.

Undisciplined

That's great, I think it's a good option to start with. They are usually schools with few students, which makes it even better because the classes are more personalized.