Is it ok to post on social media how much the PTO raised after a fundraiser?

That's a great way to share the good news! I can't think of a reason it wouldn't be OK, as long as you're not getting into the fine details of a contract with a fundraising company.

While you're sharing the good news (and thanking families for their support!), be sure to share what you can about how the fundraising proceeds will be used. Even if they're not going toward a specific project, you can use the opportunity to fill parents in on what your group does. (Our group pays for X, Y, and Z, and your support in this fundraiser goes a long way toward helping us do that.)
- egraham
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