Your student number appears on your school certificate, you will only receive your UAC number once you have gone through the registration process, with apply-by-web, you will know when you have received your UAC number because a box pops up saying "This is your UAC number! You must remember this!"
For the registration process you use your number that appears on your school certificate, and the pin that was sent to you, not the board of studies one, but the one that's actually from UAC. etc etc, they won't give you anything until you've paid.