From what I gather it sounds like you are under 18 and going on a trip with the school. From my experience with this, the chaperones need to know what meds you are taking. Sometimes there is a nurse that holds your meds and give them to you as you need them. You can't just bring your meds and take them because the school is responsible for you and trying to cut down on accidents and kids abusing drugs.

If I am right in my assumption, you do not have to tell the principal anything about your diagnosis. You only have to say that you take Lexapro. The reasons why are nobody's business but yours.