Apart from common sense, which seems to have flown out the window in certain quarters, there is a healthy body of legislation e.g. European working hours directive, rules re lorry drivers' working hours etc which recognises that nobody can be on duty 24 hours a day. It's neither healthy nor conducive to good job performance.
Teachers are not superhuman, nor are they candidates putting themselves forward for martyrdom.
Obviously school trips fall outside the normal rules in that teachers on trips are required to be present on the job 24 hours a day, and ready to step up in case of a crisis. The only other example of this that instantly springs to mind is soldiers on active duty, though there must be others. It's an unusual situation.
I know when I used to be an accountant and required to travel, my employers put me up in a nice hotel and paid me a generous daily allowance for my evening meal and any drinks I wanted as well of course as paying my salary and any overtime I did.
Yet somehow some parents on here expect teachers to work unpaid 24 hours a day with no downtime, give up their holidays and leave their families behind, not even receive a daily allowance, suck it up and don't even dream of assuming any personal autonomy as adults, such as deciding whether they'd like have a drink or two whilst other colleagues are on duty.
I honestly despair of this wonderful country sometimes, and the waggy finger brigade who are never happy unless they are telling the rest of us what to do and what to think.
O tempora, o mores!