You can redirect traffic coming to a particular port to another host on the network using xinetd. You might have to do a little tweaking for http traffic, but it is possible.
Configure your proxy server (in this reference, gateway) to redirect the traffic to the host serving http.
In my college we are using squid as a proxy server for the entire lab. We r thinking of having our own web server, but the College wants it to be behind the proxy server, Now how do I do this. I suppose this could be done by using iptables and IP forwarding, could someone please help me by giving me the code for the iptable configuration.