We have done a weeklong Washington, DC trip with 8th graders for many years. If we have fewer than 40 kids we do it with 2 teachers. The max is 53 students, as we only rent one bus for local touring -- but then we add a 3rd teacher.
We make it very clear that certain behavior gets you sent home immediately. Students are assigned 3-4 per gender segregated room, and the doors are taped at curfew. We have to actually see every person assigned to a particular room before taping. (By "tape" -- the doors are not sealed shut. We have a variety of colored or patterned tapes, and place a 3-4" strip at the very top or bottom, touching both the door and the frame. Students don't know which tape we choose each night, or where it is placed.) They are not allowed to open the door until one of us knocks in the morning. Pre-curfew visiting is only allowed with same-gender groups, and only with the permission of the chaperones -- and doors must be propped open during visitation. Chaperone rooms are on the same floor as students, and we arrange the rooms so that boys are to one side of the teachers and girls to the other.
Our schedule is so packed during the day that most of our kids are worn out by dinner time. We schedule group time in the evening, where we review the day and make required journal entries. Then we have a low-key activity, such as an ice-cream social, video and popcorn night, or swimming in the hotel pool. (We also require ALL students to wear a t-shirt over their bathing suits. No sexism here!

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If field trips are run right, they are a great addition to the curriculum. It takes a lot of work, and having chaperones who stay on top of things.
"If you think in terms of a year, plant a seed; if in terms of ten years, plant a tree; if in terms of a hundred years, teach the people." - Confucious
"Who dares to teach must never cease to learn." -- John Cotton Dana