Choosing firewall solution for school network

Bati writes: "I have to design the network infrastructure for my school as an assignment. We have to decide on which parts of the network we spend alot of money, and on which we try to keep it cheap. I wanted to use a not-so-expensive firewall solution.
Is this a good idea? And which linux solutions are available ?
You can find a diagram at this url http://bati.tripleclick.be/netwerken2007diagram.pd f
thanks,

Bati"