First, you need to figure out if you are a separate entity from the school. Some PTOs are not--they are a function of the school district, keep all their money in school accounts, and have accountability to the school district. It seems to me that if there is a business manager for your district, that person should be able to tell you if that is the case. You might also talk to people at neighboring schools' PTOs and see what they can tell you. If you're part of the school, it's not as big a problem. You just need to tell people that the wording was somewhat inaccurate, but most will be able to still work with it.
If they indicate that you are a separate and distinct organization, then find out if you have your own Federal EIN (Employer Identification Number). If you can't find it anywhere in the organization's records, try your bank; they should have one on file for you. When you get that number, call the IRS and ask them what kind of organization they have you on record as.
If you can't find any such number, and you can't find any tax returns, odds are that you are not a separate tax-exempt organization. (And if you bring in more than $5,000 or so each year, you probably should be.) So then you can either see if the school wants to accept the donations directly, or you will have to go back to the donors and just tell them that the sentence was a mistake. That may, in fact, preclude some companies from donating, because some have policies that allow them only to make donations to 501(c)(3) organizations.
With regards to writermom's comment
My understanding is, and someone please correct me if I am wrong, but donations to your PTO are not tax deductible because you are not a 501c3.
You're correct; if your PTO is not a qualified charity, the donations will not be deductible as a charitable contribution.
And as a general rule for anyone else who is reading, your sentence about deductibility should say: "Our organization is a non-profit organization under Section 501(c)(3) of the Internal Revenue Code, so your donation is
tax-deductible to the extent allowed by law. Because there are a number of situations where you cannot deduct 100% of the value of a contribution, and who knows how the law may change, so you should put that wording in there to cover your butt.