2lb a week is reasonable - you just have to make sure that you stick with the healthy lifestyle and that you aren't tempted to start skipping meals or doing excessive work-outs to achieve it.
One thing I will suggest which MAY help to you fit into your dress - get one of those miracle undergarments. I think you guys call them spanx. Anyway, they really DO work, and improve the look of the figure
I have some cheap ones - knickers that come a bit higher, and a cami that takes care of the rest. and I also have a gentle one that does a good job, and it is just a tube that goes from under the boobs to the top of the thigh. You can also get all-in-one sets.
Good luck, and enjoy yourself. Remember, your graduation is more than just a beautiful dress. If you are feeling good inside, then it was show on the outside with a beautiful glow - and that over-rides any dress :-)