My guess is you guys are running a server version 1.6.4 and trying to connect using 1.7.2.
In this case the 1.7.2 client often fails to detect the outdated server and it simply says "Can't reach server.", when it's not the actual problem.
If you guys are using the vanilla server (the one you download from minecraft.net), just make sure you're using the updated version, although if you're using CraftBukkit for that, then I suggest all the players to downgrade their clients to 1.6.4, hence CraftBukkit didn't update to 1.7.2 yet.
Hope that helps
