it's hard when it's happening to you, i know been there,
but don't let your fear stop you from doing what's best for your daughter. you never know, it may turn out to be the best decision you ever made letting her take her cup, she may look around her and decide she doesn't need it after all. i think i'd let her disover this for herself.
we tend to try and protect them so much especially at this very tender age.
my heart goes out to you.

I have to agree, I think you should let your daughter take her cup in to school, it is more important that she gets a drink at school and it is unlikely that children of 4/5 will poke fun at her, even if they do this may be just the thing to get her to stop using it and move on to more grown up cups. It doesn't however solve your problem with other drinks I guess you will just have to keep trying. This is her way of being in control. Good luck.