just go straight up to her and try talking to her la, if she is
unwilling to talk, then just ask if what thoughts she has inside
her, be it bgr, family, sch bullies, sch teachers, wanting to learn
extra stuff like 3rd language, want to work to buy lots of things
etc...
just shoot a few topic for her, and say if you can help her, then
let you know her problems, keeping it inside her will not help at
all.
if she still keep quiet, then you just observed her for a few days,
see if she got any unusual behaviours lately then decide what
you're gonna do next.
hope this helps..