@Benjispruce
Glad it's not just dd. She feels everyone else on the Durham chat is terribly excited and keen to go, and then she's the only one worried.
I did ask her if she'd put that on the chat, and she said no, so I pointed out that other people probably were similar and not saying so.
DD's very much a cat that walked by herself, and if she's decided she isn't doing fancy dress, it won't bother her being the only one without unless people start trying to persuade her to wear something else.
I don't know if matriculation varies by college, but dd's college they have to wear a gown and something "smart". Problem is smart is easy for the chaps, although I believe there was a debate going on between lounge suit and dinner suit at one point, for the girls this could be anything from ball gown to not-jeans.
I suggested a nice dress because I think a dress covers more than trousers. Only thing is, she doesn't do tights. So it needs to be a long dress. That reduces down to not many dresses. She has strong opinions on the "feel" of the fabric, even if it won't touch her skin, which takes out a fair number more, and it also needs to be warm as she feels the cold terribly!
I did get a velvet dress from ebay for not very much, which will do. Only thing is, I suspect it may run (quite bright) so have told her to make sure it doesn't go in the wash with light coloured clothes. That apparently is a distressing thought. 
She's now wondering if a trouser suit she has will do!