Well, the 2600 is a router. That is, it connects two (or more) LANs together. One of them will be the LAN in the school. The other will be the one from your ISP that is the gateway to the internet at large. It's essentially the professional version of the SOHO Linksys/Belkin thing you've probably got at home.

A big difference is that while the interface to the school's LAN is probably ethernet, the interface to the ISP's LAN is a T1 interface. The T1 comes from the phone company, probably into the white box, if it's what I think it is, a "smart jack", which is the T1 termination point that also allows the telco to remotely loop the line for diagnostic purposes. The odd thing is that I've never seen this not provided by the phone company. It's usually their equipment installed in a locked box that they don't want you to touch at all.

From there, it goes into a CSU/DSU, which is the other small box. It's essentially the T1 modem. It converts the raw T1 data into something that the router's interface can understand. I don't really know what it actually does. Probably sorts out the time division multiplexing, etc. Certainly converts it from native T1 signalling (framing and linecoding, B8ZS/ESF, probably, just to confuse you with acronyms) to serial signalling (albeit not your PC-style RS-232 serial). I've not used one in a while, nor a separate one in even longer. Cisco actually offers a T1 interface for their routers with a CSU/DSU built in, which is what I used most recently. It's not much more expensive, either, IIRC.

Then it goes into the router, which deals with the IP LAN connections, and the switch just splits the one ethernet port from the router out to multiple ones.

Hope that helps.

Edit: It's a "smart jack", not a "smartlink". Sorry. Also cleared up what a smart jack is and added some T1 protocol info.