TC my thoughts and well wishes are with you during this meeting. I know exactly how you are feeling right now because when I first became head coach there was a small movement of parents who did everything in their power to get me fired as well.
I'm going to give you some advice that someone gave me a few years ago and (in my opinion) is perfect for situations like this. At the time I couldn't appreciate it because I was a young coach and this was my first head coaching job and it would ruin my career. But looking at it now it is so true.
If this is a school and administration that will fire you over something like this then it's really not a place you want to coach at. If they are going to bow down to parents like this then at some point they will fire you for something that will be the wrong reason. If this is the case then you deserve to be in a place that will back you and support you all the way. If they can't or won't support you then getting fired is a blessing in disguise.
BC - I'm going to say tell them to make a decision and if they choose chorus over baseball then you can / need to do without them. Now I can see how I might be contradicting myself from my previous posts but I feel that a younger player can be excused for school trips but once you become a JR or SR then it's time to be depended on.