Glad to see you here in the Community, @SusanQ1.

I can help you add the tax number of your school in QuickBooks Online (QBO).

To add the tax number, I'd suggest adding a custom field on the Invoice page. Then, you'll have to manually enter the tax number in the custom field every time you create an invoice.

Here's how:

1. Go to the Gear icon.
2. Under Your Company, select Custom form styles.
3. Click the New style button, and then choose the Invoice.
4. Select the Content tab.
5. Click the Header part on the sample invoice.
6. Go to the +Custom field at the bottom.
7. Enter "Tax # of our School" as a custom name. Then, make sure you put a checkmark in the box.
8. Click Done.

You can also check out this article for more details on how to customize invoices in QBO: Customize invoices, estimates, and sales receipts.

Please also refer to this article to see different ways on how you can send invoices to your customers in QuickBooks Online: Send an invoice.

Leave me a comment below if you have other questions. I'm more than happy to help. Take care!