This used to do my head in also, these charity events that one is obliged to do.
One week near harvest festival, my kids were rooting through our food cupboards, I asked what was wrong and they said that their teachers had told them they HAD to take something in, even if it was "just a tin" or they would receive a detention and bad behaviour points.
I was livid, I can barely put meals on our table, losing 2 tins of food would be 2 meals, one for each child, or a lunch for adults or something, it was just not good enough. I sent them in with them to stop their friends picking on them, but I emailed the school and complained. I got an apology, that the teachers should not have threatened that. (which tells me they ACTUALLY did, not just that my kids were making it up)
I also hate non-uniform days, where they pay £1 each to wear non-uniform, if they don't donate they have to go in school uniform. One of these days my kids didn't tell me in advance so they went in uniform and they were bullied rotten by their peers for "not being able to afford a quid!" Again I complained, but this time it got me nowhere.
Forced donations to charity are REALLY not on.