A network bans a website either by its IP, its domain name or part of it.
When using a web proxy, your network can see only the web proxy name and address and not the actual destination of the user. A web proxy site loads the desired destination address and displays it to you as it was part of the web proxy site. The admin of the network will think everything is fine and you are free to reach the site that is banned.
The only thing that might happen is that the admin ban the web proxy as well, but this is not a big deal either. Hundreds of web proxies exist, just switch to a new one and you are ready to visit the banned site again.
