ajohnpeters, welcome to the forum.
As you can probably see from the servers.def file, there are quite a few servers available for avast to download from, and that might be a very big headache for you to manually enter all the addresses personally.
I don't know what type of "content filter" that you have setup, but there are a few that I use that work wonders with avast. I too manage schools.
Have you looked into
http://www.untangle.com/home (Free network Gateway software),
http://www.sonicwall.com (not free), or
http://www.opendns.com (free DNS service that can block inappropriate material)?
I use all three, and I'm very impressed with them all. You can always use something like Websense, if you have the funds available, but using several "layers" of the free or cheap stuff will do just as good, if not better.
Don't yell at me if you're an expert, and this came out like I was explaining these services to a 3 year old, I just don't know anything about your network yet, nor your credentials, so I'm starting from the ground-up. Hopefully something helps!