From a priest reader:
I would like to know what the appropriate academic dress is for priests at a graduation ceremony. I understand that the current "secular" dress evolved from clerical dress in the first place, namely the cassock and biretta. Would it be appropriate then for a priest to wear a cassock, with a master’s hood, and biretta? I know there is a four cornered biretta for the doctoral level, but what about for a master’s degree, or STL? Is there a three cornered biretta with red piping for theology? If I wear a traditional "graduation gown" over my suit/cassock, should I wear a biretta instead of the traditional mortar board? Thanks for your help.
Some of this will depend on where you are. Academic dress in Rome is different from academic dress in, say, England. The USA generally follows the English customs.
My understanding is that the secular/diocesan priest dresses in a house cassock appropriate to his dignity and ferraiolo with biretta. If he is holds a doctorate from a pontifical university, his four-cornered biretta is trimmed with the color of his field (theology-red, philosophy-blue, law-green). It is also possible to have a doctoral ring from Roman institutions, engraved with "Roma". To my knowledge there is no special attire for the Master’s or STL, though I have heard in some places of the three cornered biretta fo the STL on the model of the doctoral biretta. You would use the house cassock, biretta, and ferraiolo.
Depending on the tradition of the school, I suppose he could wear the doctoral robes of the institution with the academic hood. But that seems more like an English Anglican custom.
Priests often don’t know what to wear for formal occasions, such as black-tie or white-tie events. The proper house cassock and ferraiolo should be used rather than just the plain black suit.