err, no it isn't!
I am a school governor, and I can assure you that the school allocates some money to cover this, it comes from a variety of sources, and it is limited, but they do budget for covering some kids who can't pay for trips.
And I can assure you that after 30 years as a teacher/Headteacher I have paid for many many trips, lunches, coats, shoes, knickers, toiletries, bikes, cakes on cake sale days, red noses, dog food, christmas presents, bus fares, taxi fares, etc.
I think you are very naive if you think that staff don't put money into school for many many things including paying for trips.
You may allocate at your school- that doesn't mean that all schools do. It is not a requirement.