At an evening reception, there are probably a lot of people there who work all day, and if they wear suits for their jobs, then they'll be in suits there. I've been to two, and never felt out of place wearing khakis and a solid color button-down shirt.
Business casual is always safe, unless a school specifies otherwise.
This means, for guys, pressed pants (NO jeans, no cargo pants) and a button-down shirt (no logos), with leather (not tennis) shoes and trouser/dress socks. Polos are a little more informal, but if you wear a nice sweater over it, that's fine. If you're in a warmer climate, where it's a comfort factor, then a polo is probably fine.
For women, anything you'd find at Ann Taylor or a similar store qualifies. No brastraps or thongs showing. Probably want to avoid chunky heeled shoes or anything else you can't walk normally in. It won't make a good impression if you have trouble keeping your balance...
And to everyone, be nice to the olfactory receptors of people around you and keep perfumes and colognes to a minimum.