It is the great paradox of having Asperger's...while anxious and phobic in social situations, we are renown for being level-headed and calm in crisis situations where everyone else is in a panic. But since you aren't like this here are my thoughts:
* You may be traumatised (PTSD or similar) from your experiences when you were younger.
* The medical term for hypersensitive hearing is 'Auditory Hyperacusis' (or Hyperacousis, depending on which country you're in) and most Aspergers have it to some degree. But there is a related condition called 'Tonic Tensor Tympani Syndrome' (TTTS) which is an anxiety response whereby sufferers feel nauseous and anxious at the prospect that a particular noise will be painful and may harm them.
* Aspergers usually hate crowded, noisy places due to sensory problems. When the alarm goes off there is a great commotion as everyone is herded toward one or two safe areas. It may well be this you fear most...was for me.
Since the ideas listed above are all documented and recognised medical issues it shouldn't be hard to get your school to make alternative arrangements for you, provided you have one of these conditions. Go to your doctor, audiologist, occupational therapist, psychologist or whoever you see, explain your issues and get yourself tested. Once a diagnosis has been confirmed get the specialist to write a letter to your school outlining ways for you to be accommodated. If they won't listen take it to the board of education or your local politician.
Good Luck!