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You have some solid images here. I like your second set better but I think a good amount of the difference is in the post-processing. In general, I would say you could improve on:
. . . more or less in that order of concentration but 1 & 2 are close. I like your posing for the most part with the exception of a few with cropped arms.
Usually, you want the subjects face to be dominant in these types of pics. By dominant, I mean brighter or at least as bright as the rest of the elements in the frame. You want to be aware of anything else in the frame that is going to distract. For example, that fluorescent orange shirt is a real eye-puller.
Along that line, I think many of your B&W conversions have her skin/face too dark. You could correct that easy by re-doing the conversion and lightening skin tones.
You could kick these up a notch for this young lady with a small amount of skin smoothing. If you have Photoshop, here is one cool technique:
The key is to keep it subtle. Just a tiny bit is enough to gently smooth out some of the tiny blotches on her face.
You could also back off on the sharpening on a few of these. That may be accentuating some of her skin imperfections. An example is #4 in the second set. I can see sharpening halos around her hands, nose, and teeth. Her hair is also starting to take a straw-like appearance.
Zack Arias likes to repeat "head in a clean place" to his students like a mantra. It's not a bad idea at all to have that in your mind as step one to these kinds of portraits. You did great on this score in both sets with the unfortunate exception of #11 in the second set. She looks like she has an antenna coming out of her head. That is such a cool shot otherwise that it might be worth spending the time to clone out those rods in PS.
Regarding lighting, I think you just need to be more aware of how the light is falling on your subject (particularly the face). Give that as much or more priority than location and posing. You want to give some nice sculpting and dimension to the face. For a solid standard, a soft light in front of the subject slightly to the side and slightly (~45 degrees) above the subject works pretty well. Your excellent example of this is #11 in the second set.
If the light is too far to the side you get a two face effect like your #1 in set 1 (or a little bit in your tree image). There are times that that can work and provide a dramatic effect but I would use it sparingly in senior photos.
If you shoot in total shade, you can lose the dimensionality of the face unless you have a reflective source of some kind (wall, reflector, etc.). You can see this loss of dimensionality in 3, 4, and 5 of the first set. You can do some dodging and burning in post to bring some dimensionality back but it takes time.
Here is how I would judge your posted images:
Pretty great as is: #2, set 2
Pretty great with a little more post-processing: #2S1, #1S2; #3S2, #7S2, #8S2, #9S2, #10S2
Pretty great with some significant post-processing: #1S1, #3S1, #4S1, #5S1, Tree, #4S2, #5S2, #6S2, #11S2
Just to be clear, I am not promoting post-processing as the end-all and be-all. I do think it can take senior photos up a notch though if it's consistent with your style. I also think that working on post-processing while you are working on improving other aspects of your photography can be a great learning experience. It can also be a great motivator - after about the 10th time you have to spend a half-hour cloning out a distracting element from an image or making lighting adjustments, you will begin to pay more attention to those things when you take the shot.
I think your work shows a lot of promise. Keep at it.