Our school system handles this by requiring all volunteered hours and funds to go through the PTA, and then the PTA is tightly controlled for equity between schools. So, there is a list of what PTA can fund.
Inequity may come in with parent volunteers for school hours, helping teachers cut things out, etc.
But, parents cannot volunteer to help with math clubs or anything like that. Teachers can sponsor the events (for a small stipend) and then they can bring in a parent-- which is how we did it. Teacher gets paid to sit and do nothing while parents and (later) kids run the show. Of course this brings some inequity, and I hope they don't crack down on this loophole. I feel like it discourages parents from participating and a better way is to require sharing of PTA funds or inviting other schools in, but apparently that's against some major rules.
