Bring is better than take. We bring things with us when we come, and take them with us when we go. The Principal implies "Please bring a bottle of water when you come to the school picnic."
Except that today's generation seems to have forgotten "take" entirely. They never take at all, they only bring. I have tried to argue this with my children. They look at me blankly, and are just as sure that I don't know how to speak English as I am that they don't!