If you're using XP-
Go to Network Conncections, right click your wireless card, click Status, Support tab, Details, and you will find your MAC address.

In very simple terms, SSID is service set identifier. The school's network has a SSID value, and it only lets computers on the network that have this value entered. It's a security thing. The SSID will have to be entered on your wireless connection before it will let you connect.

When your laptop's wireless finds the school's network, you will have and icon in systen try (looks like 2 little computers) for the connection. Right click it and select View available wireless networks. Click to highlight the school's network, then click Advanced. In Available Networks, select the school network and click Configure. Association tab is place to enter the SSID, plus enable WEP and whatever else the school instructs you to use to be able to connect.