When companies offer trips to school, the price is scaled so that for x number of kids a teacher goes free. This seems to be standard practice in the travel trade when dealing with groups. Schools who release teachers during term time have to pay a supply teacher in their absence. Schools and local authorities are not set up to generate income from activities, other than school fetes and plays. It would take a lot of extra auditing and accounting to enable a school trip to be an earner. If a teacher incurs a legitimate agreed cost in excess of the price of the holiday, that can be rclaimed from the local authority, eg if a child fell ill and had to be accompanied back with an adult separately from the rest.
I'm not saying teachers / schools never make a few quid on the side through back-handers, but this is not an audited and accounted activity and if caught out could result in dismissal.