Internet Usage Policy

I teach computer one afternoon a week here in the local Christian school. I worked with a local ISP to get them to grant us some free internet, there are 7 computers that have access. These are also equipped with BSafe online, which is a filtering software. There is no such thing as perfect filtering software because of the nature of the internet, but anyhow I was 3 minutes late to class. I walked in and every kid in the class was playing games. I later found out that they had all been told not to play games (and download stuff).
I've talked to them about an internet usage policy, I think they should have that before they do the first thing on the internet. Maybe even have parents sign something saying that they understand the consequenses of the usage policy and the punishments for violation. I'm no so much worried about them getting on there and looking up dirty stuff as I am worried about them doing the things in plain site which will WASTE tons of time for our class and the teachers and the poor computer guy that has to maintain them. What a nightmare to have a computer lab that is full of spyware and trojans and lots of other junk that makes computers freeze up, run slow, and be less productive than they should be.