underweight for spaying

Question
I recently adopted a gs  from the shelter and she was given up by the owners for chasing horses, i noticed she has been very aggressive with people at the front door,so i took her to be evaluated and she begins classes this week the trainer said not to get her spayed right now   because she is having anxiety problem  she is under weight ,I want her spayed as soon as possible but i dont want to jepodize her health

Answer
If you have any idea when she last was in season, you have about 6 months to get it done before the next.  I hope you had her by the vet's.  I would pay a lot more attention to the vet's opinion on whether she should be spayed now.  

How underweight is underweight?  It is quite normal for young Shepherds to be on the thin side, even the ribs showing a little.  If so, and you are feeding a concentrated, meat based chow, leave well enough alone.

I am not sure about your trainer.  The key to most behavior problems is approaching things using the dog's natural instincts. Dogs see all the people and dogs in the household as a pack with each having their own rank in the pack and a top dog. Life is much easier if the 2 legged pack members outrank the 4 legged ones. You can learn to play the role of top dog by reading some books or going to a good obedience class. A good obedience class or book is about you being top dog, not about rewarding standard commands with a treat. Start at http://www.dogsbestfriend.com/ For more on being top dog, see http://www.dogbreedinfo.com./topdogrules.htm