ClamWin would not work for a school in my opinion due to the lack of an active scanner, provided that the computers in the school run windows.
I'm currently going through the same process for a car dealership, with about 80 PCs. I've recieved a quote from Kaspersky, running much less expensive than the Symantec Corporate they use now.
But, I'm thinking AVG since the packages for a client/server model AV system seem much more straight forward.