I don't think there's anything wrong with asking parents to contribute. A lot of the things this fundraising supports are 'extras' anyway, such as school trips or extra-curriculars.
As others have pointed out, there are plenty of other countries (Aus, NZ, Ire etc) where parents are expected to provide schoolbooks etc and do so happily. In the UK, people aren't used to this, perhaps, but many appear to expect 'free education' to encompass a lot more than just education itself.
I see this here - and some of the views expressed on this thread strike me as pretty entitled. Circumstances change - especially in the context of Brexit, large-scale population growth and other factors - it is unreasonable to assume that levels of funding from the Brown heyday, for example, would remain viable indefinitely.
Over the past decade, the number of primary school students has increased by more than 100,000 each year. Education spending is the second highest area of government spending after health, and each primary school student costs the taxpayer just under £5k a year. Secondary school students cost more than £6,300.
So if you have 3 children at school, their education alone is costing the taxpayer £15k per year. How much of this will your own tax bill cover?
People make vague comments about increasing government funding by cutting tax breaks for the rich or increasing corporation tax, but the reality is that many of those corporations would leave. Why do you think so many are based in Ireland, where corporation tax is so low?