This video was made for the Minnesota Historical Society in 2004. You will see a lot of the girls aren't wearing what is technically proper hijab, but more the cultural versions that are popular among Somalils.
I think it gives good insight into the variety of factors that influence a girl's decision to cover, as well as to what extent she covers.
Some of the terminology they use is Somali too.
shuuko=long slip-over khimar or sometimes an open abaya