Don't you work in a preschool? If that is right, then I can clue other posters into why any one of these might be a big deal.
The field trip shirts are so that the children and teachers can be recognized by each other in a split second. I wouldn't have gone to any extreme with the reprimand but it was a rule in our center that the teacher's wear their t-shirts on field trips. (our policy was that the center bought the shirts for the teachers so that they all had one, we did not buy them for the kids so could only ask the parents if they wanted one and explain the reason behind it but could not require the parents to purchase the shirts).
Leaving your students to go talk to another teacher? Talking to another adult is not the problem, the problem is "leaving" your students. Ratio is not debatable. You have x# of students and you will stay with x# of students. Even if they are asleep, even if they are buckled into their seats, ratio must be maintained. You are being paid to supervise the children not talk to other adults. If you can sit on the bus in a way that you can supervise your children and talk to another adult, so be it; otherwise your conversation is with the children.
Soda on the playground. Another rule we also had. The teachers could not have anything in front of the kids that the kids could not have. They had breaks and could drink whatever they wanted on those breaks. During nap time, they could have soda in their classroom. On the playground, the kids had water and so did the teachers.
Childcare centers/preschools have rules that may be a bit different than other workplaces and different than schools. As a director/owner, your first consideration has to be with the children not the comfort of the staff.
OP, your boss should be handling this in a way that doesn't single you out but there is not much you can do about it. He/she should just have a meeting and address the problems to the group; if the lead teacher still decides to break the rules, then she should be dealt with individually.
As for a "soda photo", I don't understand that--maybe a parent is trying to get rid of the teacher?