Bring her to a local gym and let her see the other women in their routines.
Make a big deal of washing bits and pits with the shower gel.
Ask her which deodorant she wants in boots, LOUDLY.
Show her the HUGE range of deodorants, explain that everyone uses them and she must too if she's acceptable in the tribe. Ask her why does she think she's not in a tribe?
Show her nature programmes of primates who isolate the sick, funny smelling ones.
Remind her we are all primates and we judge a huge amount about other primates in our group by smell. Ask her why does she think the perfume industry is so massive?
And be aware, though I'm reluctant to say it, but the start of mental health problems are signalled by the refusal to wash, and the denial or lack of awareness of smells.
Ask her questions about who else smells bad in her class, and would she sit next to a girl who smells? Does anyone else?
Tell her that no one can afford to be friends with the primate in the group who smells, as you're branded with the same brush. It's VERY urgent you get this sorted, especially as she heads into her teens and needs to be part of a same sex group.
See her GP and get a Mental health referral with her if she still refuses to smell that there is a problem. There is a big link with mental illness, specifically schizophrenia, and inability to smell a smell as everyone else experiences the smell. Research here and here
It's essential that this is sorted. I dropped a friend of mine at about 12 when I realised she stank. I couldn't afford to be friends with her. All hints fell on deaf ears, and indeed she did go on to have mental health problems. We are all primates.