To subnet or not to subnet that is the question

[Dark Lord]

I can't decide if I should subnet my work's network. I work for a small private school that's apart of a church. Currently each of the 3 offices (the church, the school, and the youth hall) are on different networks. However, we're changing our IPS to AT&T U-Verse and AT&T will only install 1 U-Verse connection per physical address. So, in order to get the U-Verse, we have to disconnect our 3 DSL connections, and get all 3 of the office building on to one network.

We just bought a Sonicwall NSA firewall, which will allow me to be subnet the network based on the ports of the firewall. The church and the youth hall only have about 6 computer stations, and the school as about 50.

I know subnetting is good idea, but we've a small environment. So I am bring more work (trouble) to myself then it's worth to subent?

I will be putting the Fellowship hall in it's own subnet/DMZ because it's an open network that we use for events; and I don't want people doing mischievous on the network.