hey..are you shooting raw or only jpeg?
if you have raw files..for the graduation, it looks like the lighting was harsh (gym?) open the raw files in the digital photo professional, and choose a different light source(there is a dropdown box with things like "tungsten, flourescent, daylight etc" choose one that corrects the white balance on those shots. you will be amazed the difference in quality when you do that. You can also add a little bit of contrast, sharpness, and saturation of the colors (dont over do that one tho as it shows quickly). Just add a little bit of each of these after you correct the white balance, and export to JPG. You can also do some Noise reduction if needed, but i think 3rd party programs are a little better. If you are only shooting jpg's, there is another program in there that can do all of these functions also, but I dont know how well it works, or anything about the functionality, so I really can't help on that one.
few hints on the portrait (from a total newb, but its the first things that jump out at me)
I know the light SUCKED, b/c you were at f2.8 1/100 @ iso 1600 (not NEAR as grainy as I would have expected tho...excellent!)
on portraits, rotate your camera 90* so you dont clip her head. You also might want her to stand a little further from the wall, and you be as close as possible, it will help blur the background a little and put the emphasis on her. The capture (expression, happiness) is excellent!
finally, try to pay attention to the light source, if you would have moved yourself to the left a few feet and made her turn, the shadows would be less. finally, this would have been a PERFECT opp for the 50 1.8 (more light, less Depth of field), I see you shot at full zoom (70mm), I think opening up to 24 and getting closer would be better in that situation (but it may have been crowded, etc) but you did great and captured shots that you can be proud of.
sanjay and buff will probably have better hints, as I believe they do some portrait/candid photos of people. Other than my son, I don't shoot peoples a whole lot (ITS TOO HARD!)