I'm not going to flog the politics of this any longer on this thread in terms of who's responsible but I do want to address ops main point.
I think that passing costs on to parents has now become the norm so much in schools that it is an accepted part of a decision making process. Schools won't put forward and idea that they haven't costed so they've obviously planned something with the intention of it being parent funded from the outset. Even if finding were increased I doubt this practice would change now. This in itself isn't a problem until it becomes an established norm, which then effectively means going forwards parents are propping up school budgets. Like any other business, once a new funding channel is established its highly unlikely to be withdrawn. However, in any other business you would never be able to plan something you didn't have a budget for and given that the school curriculum and calendar are planned so far in advance, education shouldn't be any different.
The problem is that with all the one off / ad hoc payments, there is no transparency or accountability on the part of the school as to how much they are expecting people to contribute.
In my mind the best approach, short of asking parents to pay into a monthly fund (which would be like part privatising and would not then allow people to opt out who couldn't afford it) Would be to produce a termly statement of costs and when they will be due. Has anyone added up the total cost of what you've been asked to pay? It's something I would do before then taking a proposal to the school of how to address it.
Having some kind of termly statement would allow people to plan for costs, give transparency on total amount being asked for and reduce the number of class rep requests everytime there is something new.
I also think that when it comes to PTA, a better way to approach it would be to give them a fundraising target based on previous years AND desired project costs. That way everyone knows what they're working towards and the PTA have the chance to spread the fundraisers over the year with a clear idea of how much they need to raise at each event. Likewise the school community also knows more about what's expected from them.
Lastly, in terms of time I also think this is becoming an established practice that's not going to go away. Many schools have parents involved in the school day on things like lunch duty, coming in to help change for swimming / gym lessons, reading etc... as well as volunteer hours outside school such as clas trips, pta etc.
Likewise I think adding up all these hours and letting people know on average what is expected from them will help them to plan better. There will always be some who can do more than others but as someone personally who ALWAYS does more, I'm resentful of having to prop up people who can't be bothered (not those who can't, that's different). Its got to the point that this year I'm either doing the bare minimum or nothing because I've felt that too much has been asked of me in the past simply because of my good will. I'm someone who has complicated, sad and draining family responsibilities that no one else sees and this year I'm simply not going to be made to feel like I have to justify why I'm not doing as much as I was when the person next to me could do more.
So in short (sorry for the long post), it's not going to change unless you approach it differently. To do that it needs to be measurable and the school needs to be accountable to it.
Parents money and time is a valuable resource (anyone with a brain for numbers should be able to quantify what it's worth actually) and should be treated as such. It's easy to prey on peoples feelings because it's "for the good of their children" but I do think schools need to be wary in their approach as this practice continues otherwise people's good will and resources will be quickly expended (like mine) and that will be to the detriment of all.