Agreed with the expert above!
If you want to lose 30 pounds in 4 months that equals almost 2 lbs a week.. which is not really recommended. I believe it is 1-1.5lbs a week which is healthy to loose but...
If you stick with a healthy diet and exercise regime I'm sure you can do this, or at least get close to the 130 for your prom. You will feel GREAT!
Best of luck