I believe this depends on the nature of the "donation" and if you benefit from it. See https://www.irs.gov/pub/irs-pdf/p526.pdf for details.
I'm still trying to figure this out, but my guess is
- PTA dues: I don't really get any value out of this, so I think it's OK to deduct
- Field Trips: Even though the PTA goes to great lengths to call these donations, and families who do not pay still can attend, we are basically paying for the field trip. I think this one is not OK
- Fundraisers: They offer some worthless token items to get the kids excited, but basically this is a straight donation to the school. OK to deduct.
Note that if you pay over fair value for an item at e.g. an auction, you can deduct the difference of what you paid and FMV for the item.