I'm an academic and I'm sitting on the platform for a graduation tomorrow.
It shouldn't be too hot (just as well as my gown is very very heavy and completely swamps me, as I'm rather short - I borrowed a much lighter gown for another occasion recently which was fine but it's definitely heavier than that).
I was just going to wear a blouse and trousers but saw a male member of a procession today in white tie! Now I'm wondering if I need to go more formal.
Dress by guests does not help at all as they are all dressed as if for garden party/summer wedding/ clubbing.
From memory it helps to wear something that does not ride up even when attached to a heavy gown (I cannot remember if you have to button it down but I think you do, and I remember a dreadful choice from my UG graduation where my blouse didn't tuck in and my gown kept pulling it up, but for that we had prescribed white top/black skirt and ditto my own PhD graduation had prescribed dress under the gown).
I don't actually own a gown so can't try on various outfits, I just have to go for it tomorrow morning.