But isn't some of the coat for the teachers?
Yes & no Dowsers. The company will pitch this as 'one free teacher place per 10 students' (or whatever the appropriate ratio is - 1:15 to poke around a museum or go to the theatre, 1:10 for Forn Parts generally, lower ratio for risky activities).
Now obviously they cost that in & work out that they need to charge £X per student to cover the 'free' teacher places!
However, this doesn't mean a lovely free holiday on the kids' ticket for accompanying teachers. It's bloody hard work. You don't get to sleep much & you're constantly on at least Amber alert mentally because you're responsible for hoards of other people's kids. I've done it for years & I regard each one as about the toughest week of any given year.
Plus, it's more often than not taken out of holidays, so you lose a week with your own dc (with all the obvious childcare costs & family aggro).
If it happens to take place in term time - uncommon in UK schools, but I'm international private now & we're more relaxed about that one - you have to leave a week of planning & come back to a week's worth of marking - which is actually probably worse!
I've said it before but there is nowhere on the planet I couldn't just fund a visit to by tutoring or exam marking & have it be a million times less work & stress than organising to take 50 teenagers!
Honestly this is NOT about jollies for teachers. Try asking for volunteers in the staff room & see how much begging you have to Do!