That is a huge amount, even for families where both parents work: often budgets are tight. I'd have hoped that a local theatre might put on a "school" performance, during the school day, and fill the theatre with kids for a tenner a seat.
That was a good way to do it, Gracesmum. I've a feeling we did something similar.
Teachers SHOULD go free if they are arranging a trip on behalf of the school. It is largely for the benefit of the kids, and isn't a holiday for the teacher, even if it is in an exotic place - the responsibility and organisation is huge.