I went as a parent helper on a school trip recently. Year 1 - age 5-6. I was in charge of four children one of whom would not stay with me at all, wandered off constantly. I asked him to stay with me or I would have to hold his hand which I ended up doing. I released his hand whilst looking at an exhibit and in a split second he was gone again. I had to ask the teacher to take him as it was spoiling the trip for the other children.
My point - it was MY responsibility not to lose him. The school is responsible for children on school trips. End of story. They should provide enough helpers to ensure this doesn't happen. Yes, five year olds should know that they shouldn't wander off but five is still very young and children are all different. Some are more adventurous than others, some have less knowledge of why they shouldn't wander off etc. Definitely up to the school to take responsibility and wrist bands is a ridiculous way of dealing with it IMO.