Hi all i have been having similar problems with machines on my network. Machines running windows 7 when students login they cannot access the internet, when i look into the problem im finding that the proxy is missing. It never used to be like this, as all the preferences are setup in GPO, im worried something sinister is at play
First thing I would check is to see if the user is getting the GPO (gpresult /r) and if so does the proxy settings apply if you run gpupdate while the user is logged on.
seen this happen sometimes with win 7 , when you have the proxy set in two group policies. i.e at default domain and a policy lower down. If this is the case try removing the proxy setting from the default domain policy.