This article reports:

The suspension notice states that Ryan violated a policy that says: "No student shall engage in behavior which is indecent, overly affectionate, or of a sexual nature in the school setting."

"Students leave school and essentially take the rules with them," school board lawyer Ann Majestic said in a telephone interview Friday. "The parents and the student had signed a form indicating the conditions under which a student could be off campus during the lunch hour, and it made it clear that school rules applied while the student was off campus." ...

I'm not sure that the policy is so vague that it can't be constitutionally enforced here -- much depends on the exact terms of the form -- but it seems to me that the policy's terms further suggest that the school is acting improperly. The policy itself seems to be pretty clearly focused on behavior in school or during school-sponsored events. The purpose of the policy seems to be to maintain decorum and a proper environment for learning at school. A student who's familiar with the policy would, I think, interpret it precisely this way.

If a form then says that "school rules appl[y] while the student [is] off campus," I doubt that a reasonable student would assume that this means no french kissing (might well be "overly affectionate," no?) at home or even in the car on the way to the event. I'd say that many perfectly sensible students would understand this as simply covering the things you would do while on the field trip, surrounded by students or others. That behavior in public on a school-related field trip is treated as being part of the "school setting" doesn't mean that behavior at home while playing hooky from the field trip would be treated the same way.

But even if it's not unconstitutional to apply the policy to bar sex or french kissing at home, on the theory that such behavior is barred "in the school setting" and a form says that "school rules appl[y] while the student was off campus," surely it's not quite fair -- especially when the contemplated punishment seems to be suspension for the rest of the year, which, as the news story points out, "[s]pending so much time away puts him in jeopardy of repeating 11th grade and not graduating with his class next spring."