Yes, well that would be because you can't do it.
Priority flagging is an MS Exchange sort of thing. It normally only works between two Exchange users. It's not supported in Mail at all, even if you are using an Exchange account. Very few apps, outside those meant for business Windows and Exchange users, support priority flags in email.
If there is an app for the iPad that supports it, I'm not aware of one.
If your business uses OWA (Outlook Web Access) and it is setup to support mobile devices, that might support the feature. You'd have to ask your IT department.