There was another reason why the griffin was this next event.
While Ren was a huge threat to their plan and needed to be taken care of, taking out a griffin for the sole purpose of identifying him was way too much.
Instead, they brought out the griffin just in case the worst happened.
In the end, the principal and main cast valued the lives of the students over the principal's vault.
So, if they had to choose between saving the students and the vault, the vault would be robbed while the students were fine.
In case that scenario happened, the principal decided to bring one of the most valuable things from her vault.
The griffin.
Although the main cast didn't get to see the inside of the vault, knowing the griffin was there, and not even the most valuable thing in the vault, made them even more curious about what else was inside the infamous vault.