Forget business casual. I've been employed right out of the gate at job fairs, and had interviews on the spot.
You might not be in a full business suit, but at a minimum I'd have very nice slacks, shoes and belt, a long sleeve dress shirt and tie, and a conservative sport coat. I wouldn't go any more casual than that.
I've seen others at job fairs who had on what looked like refuse from the 1970's, and I didn't see any of them leaving with a job. I was surprised people would make the effort to go to the job fair without being prepared to go for the job while they were there.
If you're going to that fair to seek an intern position in a secretaries office, by all means, wear a polo shirt. If you're going in there to seek a job as a pilot, then show up looking professional enough to get that job. It's simple, really.