Simplest way is to set up using cat5e ethernet cabling. Use a good multiport DSL router for your DSL connection, one that has Network Address Translation, DNS server and atleast 4 ethernet ports. Get atleast 2 switches (preferably 3 to have one per subnet). Each switch links direct into the DSL router via its Uplink connection to a ethernet port on the DSL router. Each switch then links to all the PCs in one of the subnets.
Set up the Office, the training and the Teen access as three seperatsub-nets. Some DSL routers have preferred local area net address ranges, if so use it. Set up each PC for its subnet and set up the router to allow each subnet access to the Internet but nor access to the other subnets.
Suggested sub net structure.
Office 10.0.1.xxx mask 255.255.255.xxx
Training 10.0.2.xxx mask 255.255.255.xxx
Teen Access 10.0.3.xxx mask 255.255.255.xxx
with the router as the default gateway.
could substitute 192.168 for the 10.0 in each case as an alternative subnetwork if you wish.
If you want further details than that you should e-mail me or send a contract (I work very cheap for community groups).