Enforce a system wide proxy in 16.04

What I want to achieve is to block adult web pages on my Ubuntu-Mate 16.04 computer. I got as far as setting up the squid proxy with squid guard on the PC. So when I use as the proxy, the filter works perfectly.

The problem is that the Firefox Connection Settings are not password protected. So an underaged users could simply change it back to "no proxy". Is there a way to enforce the usage of a proxy for the whole system that can only be changed with admin rights? (I already tried "Network Proxy" in the Mate "Control Center". But this does not seem to bother firefox.)