YABU.
It's not like the money is going to the teachers beer fund!
When talking about 'non-uniform days' and a bit of bus fare for a school trip (rather than talking cello lessons and ski trips), I find it frankly perplexing that people resent making a small outlay to enrich their DCs educational experience.
It's a school, not a fricking creche!
Yeah, they could all sit in rows and recite their times tables six hours a day.... but some kids really benefit from a chance to experience education more 'in context'.
If you're in an income bracket where you 'feel' a £5 donation for a school trip, then you would definitely 'feel' the cost of having to take yourself + DC + siblings + snack in the cafe to the same museum/show/place. I see it as a service the school provides for parents, and to level the playing field with regards to the experiences kids have had.
For example, my Dd was taken to the seaside by her school. Some of the kids there, it was the first time they'd seen the sea. The trip cost around £5-£10 i think - with a proviso about 'have a chat to the teacher if you want to pay £1 a week'.
When I was in school (in another country), we had to buy not only our uniforms and books, but also pay termly subs towards our writing paper and writing implements. Everyone did - education is a priority.