I'm in the process of setting up my network of several PCs in the classroom. All computers are configured manual IP addresses and are connected via ethernet.

Right now I want to elect one box to act as the default proxy server for surfing web pages. I know I can change settings to whatever proxy using mozilla / galeon / etc preferences in individual browsers, yet setting this up for every student on every browser is just too annoying (not to mention too hard to keep reconfiguring if students decide to change the settings back).

Surely there's a way (on Fedora Core 2) to set the default internet proxy on all machines for all users to a particular target box?

Next question: Any recommendations for antiporn filtering etc for my school which would work on Linux?

Set one computer to be the default gateway for all other computers. Basicly this computer would need 2 network cards, one going out to the internet and one going to the other computers. Your going to have to run some routing scripts but it shouldn't be too much of an issue. Also check out squid and squid guard.