wow! that's a good question!
all i can say is that whenever i've been invited to a student reception of any kind, i would dress like i would for, let's say, a wedding; according to the rules of etiquette. for this reception, and at this time of year, i'm sure a nice polo shirt and slacks would be totally appropriate for men and a nice blouse and knee length skirt or a nice dress that had the same length would be fine for ladies.
i see it this way; you may have gotten your foot in the law school door, but you can still make a lasting impression on your soon to be colleagues, professors, and staff that you'll be coming into contact with on a very regular basis. (lol)
as for the first day of class? i wish i could help you there. i would be more like you and just show up in jeans or something somewhat comfy. (lol!)
but maybe you could wear a pair of chinos/khakis and a polo to the first day of class. it's still ultra comfy and might just give you that 'official' feeling and boost your entire day along!
that's always helpful from what i've heard.
so i hope i've been helpful to you! good luck on your first year and have fun at the reception!!