What I would have been most upset about was the fact that you said no and they put her on the bus anyway. While you took an unusual stance, there are kids who don't get permission (for whatever reason) and the school needs to 1) respect that and 2) have something else for the kids who attend school that day/time period.
Permission slips are usually about more than "this is what we're doing". They also often/usually include "I give the staff permission to seek medical treatment for my child" type information, too, for which they should have your signature. It's been a while since I looked at one of my stepdaughter's slips (her dad signs them) but they are at least a half-sheet of info for even a short trip. What if the field trip required lunch or a fee or transportation across state lines (we live in a small state, so it's frequently the case), etc.? We may get a permission slip a week before a trip (if it's a short one) but generally there is a "must have or you can't go" date. If the student is not forthcoming, they don't go. The school should have had a cut-off date and contacted you then.
I would contact the school and if the school didn't have anything to say for itself, then contact the district administration. You may be known as "that mom" and maybe the boy's mom won't care, but the fact is they did what you did NOT give permission for them to do, so what was the point in asking? They shouldn't argue double negatives. You said NO.
I've also had permission slips for things like "Can this child watch an R movie for psych class?" or "Can this student attend the sex ed portion of Health class this semester?" There are a lot of reasons to ask a parent what he/she wants for his/her child. It shouldn't be lip service.
It's not always about someone else's idea of educational value or the child's wants. Otherwise the school wouldn't need to ask at all. It is NOT the principal's right to override your choice, even if he/she doesn't agree with you. It could be as simple as the trip doesn't get back til 7PM and you have another obligation.
Your child came back in one piece, but what if she hadn't? Your permission would not have been on file. Your contact info for the day wouldn't have been on file. Your permission for medical treatment (beyond life-saving measures) would not have been on file.
I also wonder, like another responder, if she was already on the bus and they were trying to cover their butts, but really it was too late. They should not have put any child on that bus without a permission slip in hand. I realize that field trip days are sometimes chaotic but have we really gotten so far from when I was a kid? It makes me wonder what other corners they are cutting.