To complement the aluminum foil on cardboard idea suggested by SJ, get yourself a couple of flat pieces of white styrofoam to redirect the light to the spots where you need it. At the hardware/construction places they have small sheets used for wall insulation. They're usually a couple of dollars and can be re-used for other shoots.
As far as poses - if she's self-conscious about her chin and nose, don't shoot from the bottom up (you lower than her face) or have her tilt her head back. Full profile also tends to highlight the nose and chin. And sure, strategically place her (or your) hands/arms for the desired effect.
To make her feel more comfortable - talk durning the shoot, keep things light, tell her there are no mistakes, have her be herself under the lights. I've found that when people are nervous/self-conscious, it helps to lead them into it gently, praise them as you go ("Nice, hold that" "That's it, let me see that smile" "OK, so someone ate your last candybar - you're upset"). Usually they get into it! It's worked with adults all the way down to pre-teens! Most of all in your situation, tell ehr you will show her the photos when you guys are done. From your assignment I gather she might not even be recognizable in the photo, so work with that.
And finally, you can surprise her and tell her you're going out to dinner/movie/coffee/shopping after the shoot
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