Thanks for the additional info; I think I've got it now.
Yes, it sounds a little fishy, and it's the book fair company that's getting ripped off. IANAL, but I do not believe that you (or anyone else in the PTO, for that matter) is responsible for any underpayment of sales taxes, for the following reason.
Sounds like the way you are set up, you're just providing a venue for this bookfair company to sell books to the kids. Although you sign a report telling them what you sold, they apparently don't check it against what inventory you return, so they have no way of knowing what books you sold for what price, or, for that matter, how many items were stolen during the fair. Someone on their end, however, is responsible for reporting gross sales to the state and remitting the appropriate sales tax. Even though they may be relying on your reports to figure out their gross sales, they have ultimate responsibility to make sure they are accurate. Maybe they are not checking very carefully because it's not material to them--underreporting $13 in sales could be a rounding error in their case--depends how big the company is. No doubt that the labor involved in checking the inventory against the sales records would far exceed $13.
Anyway, if the book fair company sets the gross sales prices, and you do not have the flexibility to modify it, then the amended report that was completed was wrong--no two ways about it. (In my earlier answer, you only would have been ok if you had the flexibility to set the sales price.)
I'm having a hard time backing into the math on this, but it sounds like it was a pretty small sale. If it were me, and the total difference was really only $25 or less, here's what I would do. I would go to the Treasurer and say something like this: "We don't have the flexibility to set the sales prices, and there must be some misunderstanding that we do, because the gross sales are less than what we actually ended up selling. I'm concerned that the report I completed was changed without consulting me, because I believe it was correct as is. I feel very strongly that we should do the right thing here, and therefore here is $25 out of my own pocket to make this right. Let's make sure this doesn't happen again. We need to either stop saying we're going to pay the sales tax, or change the way we do the final reports."
If someone came up to me and did that, I would sure take a hard look at what was going on. Should YOU really have to be out of pocket on this? Of course not. But for that relatively small amount of money, the peace of mind would be worth it. Your initial post said you were angry and hurt, and thinking about going to the school board. At the risk of offending you, I just don't think it's worth all that for a pretty small sum of money.
If my math was wrong, and the total amount owed is substantially more than $25, I would still go to the Treasurer and the new President, and go over the whole deal with them. If your Treasurer doesn't get this, you have bigger problems!
One last point--if you're not getting any profit from this deal, then your PTO budget should be showing a line for book fair expenses that is the sales tax that you are agreeing to pay for. If you can get that line item in there, then that should help eliminate any misunderstanding going forward.