Per user proxy - OS X 10.10
I have found a solution (possibly temporarily), as part of our loginhook shell script I have entered the following:

UserName="$1"
UserGroup="$(sudo -u $UserName id)"
if [[ $UserGroup == *teachers* ]] ; then networksetup -setwebproxy Ethernet &lt;hostname&gt; &lt;port&gt; off ; else networksetup -setwebproxy Ethernet &lt;hostname&gt; &lt;port&gt; off ; fi
if [[ $UserGroup == *teachers* ]] ; then networksetup -setsecurewebproxy Ethernet &lt;hostname&gt; &lt;port&gt; off ; else networksetup -setsecurewebproxy Ethernet &lt;hostname&gt; &lt;port&gt; off ; fi


Essentially it sets the proxy to a specific hostname and port with no authentication if they are part of a particular group. If they are not then they go to a standard proxy port.