Yeah the documents were stamped twice. The first stamp had the 'I certify that this is an original copy' and underneath this stamp she wrote her name and 'Assistant Principal' next to her name. The second stamp just had the school logo, contact details and address. Do you rkn it'd still be accepted as she is still technically a 'Principal'?
Yep, that's good news. It's been certified correctly. That's a good start.

That's what I thought as well. Because she technically a "principal" as you said, she should still have been able to.
In my opinion, I think that's ok... But I believe it would be best to get an official confirmation from VTAC themselves. The fastest and most efficient way is to call them on 1300 364 133 from Monday to Friday between 9-5. That way, you get a response straight away and don't have to wait 3-5 business days for a reply if you email them.