I have horrible memories of this. When I was a littleun and my parents had just split up my ma had bugger all money. We went swimming every week in school and were asked at the beginning of the term for a 'voluntary contribution.' As my ma didn't have it she wrote a note to say so and that since it was voluntary she thought that would be ok. I was stood up at the front of the class while the teacher very loudly made light of it, told me off when I cried with embarassment and hassled me and my mother so much about it for the rest of the term that she ended up paying it the next term so I could get a bit of head peace.
I had a similar argument with my daughters' school over "voluntary" contributions when they tried to hassle me and/or embarrass me into paying them after I refused. We had, in the first week of term, £4 social club fund (per family), £4 (per child) some brown envelope I can't remember what for, and they then asked for £4 (per child) voluntary contribution toward art materials as "your child will be making products they will be allowed to bring home" - this is 15 years ago and I was on benefit of £62 pw so I told them what their "voluntary" contribution added up to in food after already taking £12 of it.

For the OP: at middle school I couldn't pay for their school trips etc and the head advised me all trips were accessible to all and they had a fund for such purposes. Obviously they wouldn't be allowed to go on every trip going, but nor would any (paying) pupil, and the school not only covered the cost for the one child, but allowed me to go on the week-long holiday type trips as a parent helper which meant the other daughter got to go for free too cos I couldn't leave her alone for a week at home
