I agree that you should ask first. I went kilted to my nieces final high school play (she graduates this year) on her request. I normally leave my sgian in the car, but this time I forgot to take it out of my sock. Shortly before the play started the principal very courteously took me aside and told me that he noticed it before I was even in the doors of the school, but was not personally bothered by it as I am an adult and was in highland attire. However he continued to ask if he could put it in his office because someone else brought it up to him. He neither made me feel like a criminal or like I was going to do something stupid, but he had a job to do in the interest of public safety. He even gave it back to me well before my leaving but just asked that I put it in my jacket pocket instead of my sock. The point being that the feelings of the attendees sometimes dictate what's allowed and those in charge must be sensitive to this. Besides, unless you're wearing a doublet, a dirk may look off. That's entirely my opinion though.