Ok, so we can rule out the "i'm a leader so I have an obligation to go" reason.
Stuff what the teacher thinks (and I don't say that lightly as I'm one myself) - know that nobody can pressure you into going. The ball is not a compulsory event as it's outside school hours, so if all else fails you have that to fall back on. You cannot be made to attend something outside of school hours unless your parents are notified and have given consent for it to happen.
I think you are analysing this way too much - I am interested why you are so concerned as to what a teacher thinks/says about this issue. They have absolutely no authority to force you to go. Like I said, if it bothers you (the way the teacher is pressuring you), speak to your parents and/or somebody e.g. student coordinator.
I'm glad you can see they have the best intentions (and they probably do, not saying anything to suggest otherwise), but have you stated to them that you feel uncomfortable with what they're saying/doing? They might not even be aware of it. If you have (or don't want to tell them), refer to previous comment above re: parent/coordinator.