Ubuntu wiring suggestions
The default install without internet connection
The above picture shows a typical network setup for a single classroom without internet connection. If you follow the install notes and especially the server setup procedure this install will be the fastest and easiest.
A setup with internet connection
This is the same wiring with internet access, you need to put the right router IP in your /etc/ltsp/dhcpd.conf (as outlined in the server setup doc) for the thin clients. And make sure there is no dhcp server running on the router.
A setup with two network cards in the server
You can also route your traffic through the server as shown above if you have two network cards. This setup makes sense if you want to run a proxy server on the server machine or simply want to separate your thin client network completely from the rest of your network. Additionally to the normal server setup doc, you should also look at the NAT Howto to see how to setup the routing on the server.
Special attention to the inverters
If you use a UPS (Uninterruptible Power Supply) to supply power to the system, be careful with the distance between the UPS and the network wiring, because if they are too near, this can interfere network. For example, it can make booting the client not possible, they can't mount the nfs ltsp system, and in the clients boot process you get these errors:
mount: RPC timed out mount: NFS mount failed: bad file descriptor
and various mount fails. The client stops with a BusyBox shell. From there you can access the mounting errors issuing crl-alt-F1.
So if you get these errors, maybe you can solve the trouble installing the UPS farther from the network wiring.