You should simply tell your son he has no choice but to go on the field trip. His fear of sharks does not qualify him as a special-needs student. Suggest he closes his eyes when the class enters the shark exhibit. He most definitely should not be allowed to request that one of the adults going on the field trip stay outside the exhibit with him, and you should definitely communicate that expectation to his teacher.
More generally, one of the most counterproductive things parents can do is try to talk children out of irrational fears. The more parents talk to a child about fears — in this case, any attempt on your part to reassure your son that the sharks are fully contained and the tanks won't suddenly break and release a contagion of air-breathing, fin-walking, man-eating sharks on the city — the more likely it is that the fear will become a self-drama, a personal soap opera the child will employ to attract undue attention to himself and control various situations.
Simply tell your son, “After much thought as well as consultation with a psychologist who has devoted his career to the study and treatment of children's fears of aquarium sharks, we've decided you're going on the school trip to the aquarium. You have our permission to close your eyes when the class goes into the shark exhibit, but you do not have our permission to inconvenience your teacher or any other adult because of your fear.”
If he persists in trying to persuade you to change your mind, sit down in a comfortable chair and say, “Now that I'm comfortable, you have my permission to try your best to make me change my mind. I will listen to anything you have to say.”
After he makes his best attempt to get you to reconsider your decision, simply say, “I'm sorry, but you're just not persuasive enough. You're going on the field trip. Do you have anything else you'd like to say?”
Listen as long as need be, but keep saying “Nice try, but you're still going on the field trip.” He will give up within 10 minutes, and it will be 10 minutes well spent.
Family psychologist John Rosemond answers parents’ questions on his web site at www.rosemond.com.