Sigh. I had to beg my mom to let me get a training bra (and endure her saying, "but you have no breasts") because the boys were snapping bras and I didnt' have one. :( I may have reached an A by graduation...
What she needs is the rock solid self esteem to wear her frame with pride and fend off all untoward onslaughts with a rapier wit and a nonchalant air. But. What 13 year old has that? NONE.
I say honor her modesty, don't make her take off jackets etc, let her wear what she wants (be glad she's not insisting on bursting out of low cut tops like some teens), but do encourage her not to be ashamed. Tops that FIT without being tight, etc. I hope you get some great advice from ladies who have been there. I certainly wouldn't know what it's like. :(
I agree with Riley, if she can get the gumption to call people out for being inappropriate by turning the ridicule on them for not being able deal with boobs, it will scare off a lot of the bad behavior. "I'm up here" would go a long way for the boys with their eyes on her chest. Sometimes they ogle to be "funny" (because for real, anyone CAN control that) if there is no resistance, but feel stupid if they get called on it. Remember, bullies and rude people usually prey upon the weak. So she needs a thick skin and bravery to speak up.
Oh, yeah, and keep her focused on the good qualities about her that DO matter!