well, first off, if you don't have anywhere you can play on wifi I don't know why you need your code. But whatever.
Anyways, not every wifi access point is created equal. On my home setup I had to switch my Wifi to 2mbps only for it to work with my DS. So maybe the access point you were trying to use was incompatible. There isn't much you can do in this situation, as I'm sure petitioning them to change the Wifi just so you can play DS will get you nowhere 
So, assuming you don't actually want to play in the cafeteria, go wherever you will be playing (say a friends house) and try using their router. Since it is in their house, if you can't connect go to this page - http://www.nintendo.com/consumer/wfc/en_na/ds/routerInfo.jsp - and then it will tell you how you need to have the router set up. (how I figured out my router had to be set to 2Mbps)








