[BullyPreventionScheme]

hmmmm, so telling the teachers doesn't work, so thats that option is erased.

Hmmm but you said before she can't stick up for herself, which means she has a lack of confidence. And thats probably why the "not let it bother her" act isn't working, she probably still looks sad even though shes trying not to.

Ok, two things so far which i see you can do.

1. Her confidence needs be lifted first, so try to get her inolved in something that puts her out of her comfort zone. Also no thanks to all that bullying she probably has a pretty poor body image. So do things that make her feel good like:
Put ribbons in her hair and say she cute. Oh, and if she goes onto the topic of her weight, say it makes her cute and cuddly. when dealing with bullies its important to make flaws special.

2. Now that you have raised her confidence she can stop those bullies, make sure she knows these 3 things:
Always Keep eye contact with a bully, (it brings down their confidence)
Always keep a smile on your face no matter what. (yet again brings down their confidence.)
When shes walking around tell her to keep her back straight and her chin high. (this stops bullies from even talking to you in the first place.)
When they make a joke about her weight, laugh like she finds it funny. (bullies will retreat. when they think there not hurting her.) If they ask why shes laugh, get her to say: because its funny. (ignorant answers can really screw bullies up.)

If she keeps that up the bullying will slowly stop. See if you can get her to practice it infront of you. (it should help with confidence)

If you have any questions, please, ask away.