It is courtesy to dress appropriately, so I think a tie is in order for this kind of turn-out.
Depending on the size of the student body, you sould allow one to two hours for the ceremony itself, and there are usually photos and a chat afterwards.
It's like school plays, prize days, concerts, and so on. You go along, get a sore behind from sitting too long on a hard chair, feel proud for the few minutes that your family member is involved, and spend the rest of the time fidgeting and quietly cursing under your breath but ...
it's the same for everyone!
it's a couple of hours out of your life.
it's important to your grand-daughter to be there and be proud.
it won't kill you to put yourself out for someone else.
OK?
Good.