I think that the idea that we can cast Patronus charms or not is moot due to the fact we each were required to list what ours resembled.
We are the PCs in the RP and as such should stand a little outside the norm

This is the reconstructed Hogwarts following the Wizarding Wars and in the final battle there were many students performing spells outside of what they'd been taught as the standard curriculum because of Dumbledor's Army training. I could easily justify why the school would teach students more advanced spells if they show an aptitude for casting them.