Karin-
I have a snide woman like that at my son's front desk at his school, too. It makes me wonder why someone would even choose a career like that if they can't seem to be nice, and how in the world they got the job in the first place!
The fact of the matter is, she is not doing her job properly, and as such, should be reported to her supervisor. I would suggest you call the principal and complain. Nothing you say to people like this is going to stop them, but maybe a good talking to from her boss would.
When you talk to the principal, be as nice as possible, but BE FIRM! Let him know that you find this behavior unacceptable and that you will not tolerate it in the future.
I hope this suggestion helps, and the principal puts a stop this woman's ridiculous behavoir. Neither you nor your son should have to deal with derision at his school, ESPECIALLY since you're dealing with a health issue. DISCRIMINATION is not acceptable in any form.
P.S. I'd be willing to be you won't be the first parent to complain about her.
L.