Dear Julie,
What a great opportunity! It's really important to start with the best photos you can...
That being said, I think you should take more photos. In the first, I can see that there is a fish-eye distortion caused by not being far enough back from your subject to zoom in.
I think you can take photos that would flatter your subject more- sitting up straight and leaning forward, where you are a bit above eye-level. She will look more slender and more dancer- like this way, I believe.
The red white and blue scheme is not relevant since the ad will be in black and white - but the value pattern IS. The RWB scheme ( and the third) is one that results in isolate chunks of dark and middle values. Whatever images you are considering, look at them in black and white/grayscale mode.
Think also about the size of the image in the ad, and how it will be printed. Even at 8 1/2 x 11, there's not much room for a head, let alone a half figure or full figure pose. Also find out about the quality of paper, and whether the magazine is going to a printer, or will be created by a Xerox machine. The poorer the quality of the outcome print, the more important it is to have an image that will read well in that context - small heads that are Xeroxed will be nearly indistinguishable. So here you see me arguing for a head/shoulders or h/s with hands, unless your ad will be printed properly on high quality stock . You might experiment with dance shoes tied and over the shoulder something like that. The dancer's hair and leotard will convey a lot, after all, it's in a dance magazine
Also, you should move your light source to a more oblique position. It's so head on that you are not left with much light/shadow information to convey the form.
Good luck on this exciting opportunity!