I am sometimes the person at my school who makes decisions, including decisions that might upset parents or teachers.
The first thing I'll say is that the other administrators at my school, and I, put a lot of thought into the decisions we make. That isn't to say that we're always right, but we are always thoughtful, and generally have the best interest of the kids in the forefront of our minds. If someone wants to come in and discuss a decision I've made, I'm happy to have that conversation. Why knows what will happen? You might listen to my reasons and come around to my side. You might convince me to look at the situation in a different way and change my mind. We might find a third solution that works for both of us. Whatever, but it starts with approach admin as if they're intelligent partners.
As to the specifics of the zoo trip, all I have to say is this: Do you have a parent in your community you'd never trust with your kid? Maybe you've heard the way he yells at his wife and it makes your skin crawl. Maybe you've found her toddler wandering in the street one time too often. Maybe you're creeped out by the way he looks at your preteen daughter. Or maybe she's come to pick up her 5 year old with alcohol on her breath many many times. For whatever reason, you've made a decision not to let him/her be alone with your kids.
Well, that neighbor's kids go to school, and their parents chaperone too. As a teacher, I can tell you that I put a lot of thought into how I group kids and chaperones on field trip. I'll take whatever steps I need to make sure that so and so isn't alone with your kids, whether it's rigging the lottery for spots on the bus, or assigning that parent to share a group with me or with my most trustworthy assistant teacher. But on a field trip where parents can come and go freely, it's a lot harder to manage. What if that parent runs into another parent group before you see them, and offers to take a couple kids to the bathroom? And that's just one example.
That isn't to say that there might not be a solution that would make you and the school happy. Maybe if parents sign up in advance, come to the school, attend the safety presentation, and then carpool behind the bus, it can work. Maybe parents can raise money and rent a second bus. If this is really really important to you then a conversation is a good place to start. On the other hand, it's totally reasonable to decide to just live with a polcy that you don't love.