"What I am still (unsuccessfully) trying to fathom is why any woman would actively choose to wear something that has become a symbol of oppression"
Well it is a symbol of oppression to Western perceptions not to all perceptions.
I also as another poster mentioned have come across various instances of family members NOT wanting the women in their family to wear the hijab but they do anyway. Many of my friends in the last few years have started wearing the hijab and their parents have at first been mortified so they havent been forced.
In terms of uniform, well it is a Muslim school, so it kind of makes sense.
My little SIL enjoys matching her hijab to her clothes and things and wears pretty little hat pins and brooches to secure it. So the argument about cultural fashion also holds so value.